Krystal @ Chisholm's Blog

November 26, 2010

Chapter 9 – Troubleshooting Methodologies and Tools

Filed under: Uncategorized — krystalchisholm @ 11:10 am

The OSI Model and Troubleshooting

One of the most important abilities for a network professional to develop is the ability to efficiently troubleshoot network problems. Good network troubleshooters are always in high demand. For this reason, Cisco certification exams measure the ability to identify and correct network problems.

When troubleshooting, many technicians use the OSI and TCP/IP networking models to help isolate the cause of a problem. Logical networking models separate network functionality into modular layers. Each layer of the OSI or TCP/IP model has specific functions and protocols. Knowledge of the features, functions, and devices of each layer, and how each layer relates to the layers around it, help a network technician to troubleshoot more efficiently.

This chapter uses the OSI and TCP/IP models to provide the structure for troubleshooting activities. Before beginning, review the material on the OSI and TCP/IP models in CCNA Discovery: Networking for Home and Small Businesses and CCNA Discovery: Working at a Small-to-Medium Business or ISP.

OSI Reference Model as a Troubleshooting Tool

The OSI reference model provides a common language for network technicians and engineers. It is important to understand the functions that occur and the networking devices that operate at each layer of the OSI model.

The upper layers (5-7) of the OSI model deal with specific application functionality and are generally implemented only in software. Problems isolated to these layers can frequently be caused by end-system software configuration errors on clients and servers.

The lower layers (1-4) of the OSI model handle data-transport issues.

The Network Layer (Layer 3) and the Transport Layer (Layer 4) are generally implemented only in software. In addition to software errors on end systems, software configuration errors on routers and firewalls account for many problems isolated to these layers. IP addressing and routing errors occur at Layer 3.

The Physical Layer (Layer 1) and Data Link Layer (Layer 2) are implemented in both hardware and software. The Physical Layer is closest to the physical network medium, such as the network cabling, and is responsible for actually placing information on the medium. Hardware problems and incompatibilities cause most Layer 1 and Layer 2 problems.

Troubleshooting Methodologies

There are three main troubleshooting approaches when using network models:
– Top-down
– Bottom-up
– Divide-and-conquer

Each method assumes a layered concept of networking. Using one of these troubleshooting methods, a troubleshooter can verify all functionality at each layer until the problem is located and isolated.

Top-down – Starts with the Application Layer and works down. It looks at the problem from the point of view of the user and the application. Is it just one application that is not functioning, or do all applications fail? For example, can the user access various web pages on the Internet, but not email? Do other workstations have similar issues?

Bottom-up – Starts with the Physical Layer and works up. The Physical Layer is concerned with hardware and wire connections. Are cables securely connected? If the equipment has indicator lights, are those lights on or off?

Divide-and-Conquer – Typically troubleshooting begins at one of the middle layers and works up or down from there. For example, the troubleshooter may begin at the Network Layer by verifying IP configuration information.

The structure of these approaches makes them ideally suited for the novice troubleshooter. More experienced individuals often bypass structured approaches and rely on instinct and experience.

Troubleshooting Tools

It is very difficult to troubleshoot any type of network connectivity issue without a network diagram that depicts the IP addresses, IP routes, and devices, such as firewalls and switches. Logical and physical topologies are extremely useful in troubleshooting.

Physical Network Topologies

A physical network topology shows the physical layout of the devices connected to the network. Knowing how devices are physically connected is necessary for troubleshooting problems at the Physical Layer, such as cabling or hardware problems. Physical network topologies typically include:
– Device types
– Models and manufacturers of devices
– Locations
– Operating system versions
– Cable types and identifiers
– Cabling endpoints

Logical Network Topologies

A logical network topology shows how data is transferred on the network. Symbols are used to represent network elements such as routers, servers, hubs, hosts, and security devices. Logical network topologies typically include:
– Device identifiers
– IP addresses and subnet masks
– Interface identifiers
– Routing protocols
– Static and default routes
– Data-link protocols
– WAN technologies

In addition to network diagrams, other tools may be needed to effectively troubleshoot network performance issues and failures.

Network Documentation and Baseline Tools
Network documentation and baseline tools are available for Windows, Linux, and UNIX operating systems. CiscoWorks can be used to draw network diagrams, keep network software and hardware documentation up to date, and help to cost-effectively measure baseline network bandwidth use. These software tools often provide monitoring and reporting functions for establishing the network baseline.

Network Management System Tools
Network Management System (NMS) tools monitor network performance. They graphically display a physical view of the network devices. If a failure occurs, the tool can locate the source of the failure and determine whether it was caused by malware, malicious activity, or a failed device. Examples of commonly used network management tools are CiscoView, HP Openview, SolarWinds, and WhatsUp Gold.

Knowledge Bases
Network device vendor knowledge bases have become indispensable sources of information. When online knowledge bases are combined with Internet search engines, a network administrator has access to a vast pool of experience-based information.

Protocol Analyzers
A protocol analyzer decodes the various protocol layers in a recorded frame and presents this information in a relatively easy-to-use format. Protocol analyzers can capture network traffic for analysis. The captured output can be filtered to view specific traffic or types of traffic based on certain criteria; for example, all traffic to and from a particular device. Protocol analyzers, such as Wireshark, provide detailed troubleshooting information about the data being communicated on the network. An example of the types of information that can be viewed using a protocol analyzer is the setup and termination of a TCP session between two hosts.

Sometimes failures in the lower layers of the OSI model cannot be easily identified with software tools. In these instances, it may be necessary to use hardware troubleshooting tools, such as cable testers, multimeters, and network analyzers.

Cable Testers
Cable testers are specialized, handheld devices designed for testing the various types of data communication cabling. Cable testers can be used to detect broken wires, crossed-over wiring, shorted connections, and improperly paired connections. More sophisticated testers, such as a time-domain reflectometer (TDR), can pinpoint the distance to a break in a cable. Cable testers can also determine the length of a cable.

Digital Multimeters
Digital multimeters (DMMs) are test instruments that directly measure electrical values of voltage, current, and resistance. In network troubleshooting, most of the multimeter tests involve checking power-supply voltage levels and verifying that network devices are receiving power.

Portable Network Analyzers
By plugging a network analyzer into a switch anywhere on the network, a network engineer can see the average and peak utilization of the segment. The analyzer can also be used to identify the devices producing the most network traffic, analyze network traffic by protocol, and view interface details. Network analyzers are useful when troubleshooting problems caused by malware or denial-of-service attacks.

Layer 1 and 2 Problems

The Physical and the Data Link Layers encompass both hardware and software functions. All network communications rely on the technologies at these layers to function. A network technician must be able to quickly isolate and correct problems occurring at these layers.

The Physical Layer, or Layer 1, is responsible for the physical and electrical specifications for the transmission of bits from one host to another over the physical medium, either wired or wireless. Network problems occurring at Layer 1 can cause the loss of network connectivity, or simply cause network performance to degrade.

The types of problems that occur at Layer 1 are directly related to the type of technology used. For example, Ethernet is a multi-access technology. Ethernet protocols use an algorithm to sense when there are no other signals on the wire to begin a transmission. However, it is possible for two devices to begin sending at the exact same time, causing a collision. When a collision occurs, all devices stop transmitting and wait a random amount of time before transmitting again. Because Ethernet can detect collisions and respond to them, Ethernet is often referred to as Carrier Sense Multiple Access with Collision Detection (CSMA/CD).

However, excessive collisions can cause network performance to degrade. Collisions can be a significant problem on shared media, such as a hub network, more so than on switched ports.

The Data Link Layer, or Layer 2, specifies how the data is formatted for transmission over the network media. It also regulates how access to the network is granted. Layer 2 provides the link between the Network Layer software functions and the Layer 1 hardware for both LAN and WAN applications. To effectively troubleshoot Layer 1 and Layer 2 problems, technicians must be familiar with cabling standards, and encapsulation and framing.

After a technician verifies that Layer 1 is functioning, it must be determined if the problem resides in Layer 2 or one of the higher layers. For example, if a host can ping the local loopback address,, but cannot access any services over the network, the problem may be isolated to Layer 2 framing issues or a misconfigured interface card. Network analyzers and other online tools can locate the source of a Layer 2 issue. In some instances, a device recognizes that a Layer 2 problem occurred and sends alert messages to the console.

Troubleshooting Device hardware and Boot Errors

Network problems often occur after a device is restarted. Restarts can happen intentionally after an upgrade, or unexpectedly after a power failure. To troubleshoot device hardware failures and boot errors, it is first necessary to review the process that Cisco IOS devices use during startup. The bootup process has three stages:

1. Performing the POST and loading the bootstrap program.
2. Locating and loading the Cisco IOS software.
3. Locating and loading the startup configuration file or entering setup mode.

When booting any Cisco networking device, it is helpful to observe the console messages that appear during the boot sequence. After the Cisco IOS software is loaded, the technician can use commands to verify that the hardware and software are fully operational.

The show version command displays the version of the operating system and whether all interface hardware is recognized.

The show flash command displays the contents of the Flash memory, including the Cisco IOS image file. It also displays the amount of Flash memory currently being used and the amount of memory available.

The show ip interfaces brief command shows the operational status of the device interfaces and IP addresses assigned.

The show running-configuration and show startup-configuration commands verify whether all the configuration commands were recognized during the reload.

When a device fails to boot correctly and creates a network outage, replace the device with a known good device to restore services to end users. After service is restored, then take the time to troubleshoot and repair the failed device.

After a router boots successfully, the green LED indicators will display. When errors occur during the bootup process, Cisco devices execute default actions to recover from the errors, such as loading into ROMmon mode. There are five common bootup errors (discussed on this page and the next), that have associated troubleshooting strategies.

Device Fails POST
When a device fails POST, no output appears on the console screen. In addition, system LEDs may change color or blink, depending on the device type. For a description of LED operation, check the documentation provided with the device. If the POST fails, turn off the power, unplug the device, and remove all interface modules. Then reboot the device. If the POST still fails, the device requires service. If it completes the POST successfully without the interface modules installed, an interface module may have failed. Disconnect the power and reinstall each module individually, rebooting each time, to determine which module has failed. When the failed module is identified, replace it with a known good module and restart the device.

Cisco IOS Image in Flash is Corrupt
If the image file in flash is corrupt or missing, the bootloader cannot find a valid Cisco IOS file to load. Some Cisco IOS devices have an image with limited functionality that is loaded and run if no image exists in flash or another specified location. This image is called a boothelper. Boothelper images may not have enough functionality to successfully execute the necessary configuration commands to bring the device back into operation. If there is no boothelper, the device enters ROMmon mode. Use ROMmon commands to reload the correct Cisco IOS image from a TFTP server.

Memory is not Recognized or Fails
If there is not enough memory to decompress the image, the device scrolls error messages rapidly or constantly reboots. The device may be able to boot into ROMmon mode by issuing a Ctrl-Break command during startup. In ROMmon mode, commands can be issued to determine the status of the memory. The memory may have to be replaced or increased for the device to function normally.

Interface Modules are not Recognized
Faulty or improperly seated interface modules may not be recognized during the POST and Cisco IOS load. When this occurs, the list of available interfaces displayed by the show version command does not match the physically installed modules. If an interface module is new, check that the module is supported by the Cisco IOS version that is installed and that enough memory exists to support the module. Always power down the device, disconnect the power, and reseat the module into the device to determine if there is a hardware problem. After reseating, if the module is not recognized during reboot, replace it with a known good module.

Configuration File is Corrupt or Missing
If a valid startup configuration file cannot be found, some Cisco devices execute an autoinstall utility. This utility broadcasts a TFTP request for a configuration file. Other devices immediately enter an initial configuration dialog, known as the setup utility or setup mode. Devices that have the autoinstall utility also enter setup mode if no TFTP server responds after five inquiries. Use either TFTP or manual configuration to reload or recreate the configuration. Devices do not forward traffic until a valid configuration is loaded.

Troubleshooting Cable and Device Port Errors

Router interface errors are often the first symptom of Layer 1 and Layer 2 cabling or connectivity errors. To troubleshoot, begin by examining the statistics recorded on the problematic interface using the show interfaces command and the status of interfaces using the show ip interface brief command.

The output for the show ip interface brief command includes a summary of the device interfaces, including the IP address and interface status.
Up/up status – indicates normal operation and that both the media and the Layer 2 protocol are functional.
Down/down status – indicates that a connectivity or media problem exists.
Up/down status – indicates that the media is connected properly, but that the Layer 2 protocol is not functioning or is misconfigured.

Common cable or media issues that can cause a down/down output include:
Loose cable or too much tension on the cable – If all the pins cannot make a good connection, the circuit is down.
Incorrect termination – Ensure that the correct standard is followed and that all pins are correctly terminated in the connector.
Damaged serial interface connector – Pins on the interface connection are bent or missing.
Break or short in the cable – If there are problems along the circuit, the interface cannot sense the correct signals.

Common Layer 2 issues that can cause an up/down output include:
– Encapsulation is improperly configured.
– No keepalives are received on the interface.

Occasionally, media errors are not severe enough to cause the circuit to fail, but do cause network performance issues. The show interfaces command provides additional troubleshooting information to help identify these media errors.

Output for the show interfaces command includes:
Excessive Noise – On Ethernet and serial interfaces, the presence of many CRC errors but not many collisions is an indication of excessive noise. CRC errors usually indicate a media or cable error. Common causes include electrical interference, loose or damaged connections, or using the incorrect cabling type.
Excessive collisions – Collisions usually occur only on half-duplex or shared-media Ethernet connections. Damaged cables can cause excessive collisions.
Excessive runt frames – Malfunctioning NICs are the usual cause of runt frames, but they can be caused by the same issues as excessive collisions.
Late collisions – A properly designed and configured network should never have late collisions. Excessive cable lengths are the most common cause. Duplex mismatches can also be responsible.

Troubleshooting LAN Connectivity Issues

LAN troubleshooting usually centers on switches, because the majority of LAN users connect to the network via switch ports. Many of the same Cisco IOS show commands can be used on switches to gather troubleshooting information. In addition, each port on a switch has an LED indicator that provides valuable troubleshooting information.

The first step in troubleshooting LAN connectivity issues is to verify that the switch port connected to the user is active and that the appropriate LED indicators are lit. If there is physical access to the switch, it can save time to look at the port LEDs, which give the link status or indicate an error condition (if red or orange). Check to see that both sides of the connection have a link.

If no link light is present, ensure that the cable is connected at both ends and that it is connected to the correct port. Make sure that both devices are powered up, and that there are no bootup errors on either device. Swap out any patch cables with known good cables and verify that the cable terminations are correct for the type of connectivity desired. If there is still no link light, verify that the port is not administratively shut down. Use the show running-config interface command to show the parameters configured on a switch port:
Switch#sh run interface fastEthernet 4/2
interface FastEthernet4/2
duplex full
speed 100

Even if a link light is present, it does not guarantee that the cable is fully functional. The cable can be damaged, causing intermittent performance problems. Normally, this situation is identified by using Cisco IOS show commands to determine if the port has many packet errors, or if the port constantly flaps (loses and regains a link).

The show version and show interfaces commands executed on a switch provide similar information to the same commands executed on a router. To get a quick view of switch port error statistics, use the show interface port counters errors command.

Duplex mismatches are more common on switches than on routers. Many devices are set to autonegotiate speed and duplex settings. If one device on a link is configured to autonegotiate and the other side is manually configured with speed and duplex settings, mismatches may occur, leading to collisions and dropped packets.

To view the speed and duplex settings on a port and whether manual or autonegotiation features were used, use the show interface port status command.

If the mismatch occurs between two Cisco devices with the Cisco Discovery Protocol (CDP) enabled, there are CDP error messages on the console or in the logging buffer of both devices. CDP is useful to detect errors and port and system statistics on nearby Cisco devices.

To correct duplex mismatch errors, set both devices to autonegotiate speed and duplex. If the negotiation does not produce the desired results, manually configure matching speed and duplex settings on each device.

Troubleshooting WAN Connectivity Issues

Troubleshooting a serial WAN connection is different from troubleshooting Ethernet LAN connections. Typically, WAN connectivity relies on equipment and media that is owned and managed by a telecommunications service provider (TSP). Because of this, it is important for technicians to know how to troubleshoot the customer premises equipment and to communicate the results to the TSP.

Most serial interface and line problems can be identified and corrected using information gathered from the show interfaces serial command. Serial connections may experience problems caused by packet errors, configuration errors, or mismatches in encapsulation and timing. Because serial WAN connections usually rely on a CSU/DSU or modem for timing, these devices must be considered when troubleshooting serial lines. In prototype networks, a router can be configured to provide DCE clocking functions, eliminating the CSU or modem.

To successfully troubleshoot serial WAN connectivity problems, it is important to know the type of modem or CSU/DSU that is installed and how to place the device in a loopback state for testing.

The interface status line of the show interfaces serial command can display six possible problem states:
Serial x is down, line protocol is down (DTE mode) – When the router serial interface cannot detect any signal on the line, it reports both the line and the Layer 2 protocol down.
Serial x is up, line protocol is down (DTE mode) – If the serial interface does not receive keepalives or if there is an encapsulation error, the Layer 2 protocol is reported down.
Serial x is up, line protocol is down (DCE mode) – In cases where the router is providing the clock signal and a DCE cable is attached, but no clock rate is configured, the Layer 2 protocol is reported down.
Serial x is up, line protocol is up (looped) – It is common practice to place a circuit in a loopback condition to test connectivity. If the serial interface receives its own signals back on the circuit, it reports the line as looped.
Serial x is up, line protocol is down (disabled) – High error rates cause the router to place the line in a protocol disabled mode. This type of problem is usually hardware related.
Serial x is administratively down, line protocol is down – An administratively down interface is one that is configured with the shutdown command. Usually all that is needed to fix this condition is to enter the no shutdown command on the interface. If the interface does not come up using the no shutdown command, check the console messages for a duplicate IP address message. If a duplicate IP address exists, correct the problem and issue the no shutdown command again.
Serial x is up, line protocol is up – The interface is operating as expected.

Review of Layer 3 Functionality and IP Addressing

Layer 1 networks are created by interconnecting devices using physical media. Layer 2 network protocols are hardware dependent. Ethernet cannot operate over a serial link, nor can serial communications occur using an Ethernet NIC.

Layer 3 (the Network Layer) protocols are not bound to a specific type of media or Layer 2 framing protocol. The same Layer 3 protocols can operate on Ethernet, wireless, serial, or other Layer 2 networks. Layer 3 networks can contain hosts that are connected using different Layer 1 and 2 technologies. The primary functions implemented at Layer 3 of the OSI model are network addressing and routing. Layer 3 networks are referred to as logical networks because they are created only in software.

Today most networks implement the TCP/IP protocols to exchange information between hosts. As a result, much of the focus of troubleshooting Layer 3 problems is concentrated on IP addressing errors and on routing protocol operation.

Troubleshooting Layer 3 problems requires a thorough understanding of network boundaries and IP addressing. Poorly designed and configured IP addressing schemes account for a large number of network performance problems.

At Layer 3, each packet must be identified with the source and destination addresses of the two end systems. With IPv4, each packet has a 32-bit source address and a 32-bit destination address in the Layer 3 header.

The IP address identifies not only the individual host, but also the Layer 3 local network on which the host can communicate. A simple IP network can be created by configuring two interconnected hosts with unique addresses that share the same network prefix and subnet mask.

A device must be configured with an IP address to exchange messages using TCP/IP. Individual Layer 3 IP networks encompass a range of IP addresses. These boundaries are determined by the number of bits contained in the network prefix portion of the address. A simple rule is the longer the network prefix, the smaller the range of IP addresses that can be configured on hosts in that IP network.

To troubleshoot Layer 3 problems, an administrator must be able to determine the range of host addresses that belong to each individual IP network. The range of addresses is determined by the number and position of host bits. For example, in a network, borrow three bits for subnetting. This leaves 5 bits for host addresses. This creates 8 subnets (2^3=8) and 30 hosts per subnet (2^5 – 2 = 30).

Given the subnet, the first host on the subnet will be, and the last host will be The broadcast address for this subnet will be This can be seen by looking at the binary of the last octet:

(011 subnet) 96 + (00001 first host) 1 = (01100001) 97 in decimal

(011 subnet) 96 + (11110 last host) 30 = (01111110) 126

(011 subnet) 96 + (11111 broadcast) 31 = (01111111) 127

This example is using a class C address. This same technique can be applied to Class A and Class B addresses. Remember that the location of host bits can extend into more than one octet.

IP Design and Connectivity Issues

If IP addressing is assigned in a random manner, it is difficult to determine where a source or destination address is located. Today, most networks employ a hierarchical IP addressing scheme. Hierarchical IP addressing schemes offer many advantages, including smaller routing tables that require less processing power. Hierarchical IP addressing also creates a more structured environment that is easier to document, troubleshoot, and expand.

However, a poorly planned hierarchical network, or a badly documented plan, can create problems, such as overlapping subnets or incorrectly configured subnet masks on devices. These two conditions account for many IP addressing and routing issues within networks.

An overlapping subnet occurs when the address range of two separate subnets include some of the same host or broadcast addresses. Overlapping is usually a result of poor network documentation or by accidentally entering the incorrect subnet mask or network prefix. Overlapping subnets do not always cause a complete network outage. They may only affect a few hosts, depending on where the misconfigured subnet mask is placed.

Cisco IOS software does permit you to configure an IP address from overlapping subnets on two different interfaces. However, the router does not activate the second interface.

For example, the router R1 interface Fast Ethernet 0/0 is configured with an IP address and subnet mask on the network. If Fast Ethernet 0/1 is configured with an IP address on the network, an overlapping error message appears. If there is an attempt to enable the interface with the no shutdown command, a second error message appears. No traffic is forwarded through the interface. The output from the show ip interface brief command shows that the second interface configured for the network, FastEthernet 0/1, is down.

It is important to verify the status of the interfaces after making configuration changes. An interface that remains administratively down after the no shutdown command is issued can indicate an IP addressing problem.

Although Cisco IOS software has safeguards to ensure that overlapping subnets are not configured on multiple interfaces of the same device, it does not prevent overlapping subnets from being configured on different devices or on hosts within the network.

A poorly configured subnet mask can cause some hosts on a network to not have access to network services. Subnet mask configuration errors can also present a variety of symptoms that may not be easily identified.

IP Address Planning and Allocation Issues

Poor address allocation planning can cause other problems. Often, an administrator underestimates the potential for growth when designing subnets. As a result, the IP subnetting scheme does not allow for enough host addresses in each subnet. One indication of a subnet having too many hosts is when some hosts are unable to receive an IP address from the DHCP server.

When a host running Microsoft Windows does not receive an address from a DHCP server, it automatically assigns itself an address on the network. If this occurs, use the show ip dhcp binding command to check whether the DHCP server has available addresses .

Another indication of not enough IP addresses is an error message on a host stating that duplicate IP addresses exist. If a host device is turned off when the DHCP lease expires, the address is returned to the DHCP pool and can be issued to another host. When the original lease holder is turned back on, it requests a renewal of its previous IP address. In a Microsoft Windows network, both hosts report a duplicate IP address error.

DHCP and NAT Issues

DHCP can create another level of complication when troubleshooting network issues. If hosts are configured to use DHCP and are not able to connect to the network, verify that IP addressing is assigned using the Windows command, ipconfig /all. If hosts are not receiving IP addressing assignments, it is necessary to troubleshoot the DHCP configuration.

Regardless of whether the DHCP service is configured on a dedicated server or on the router, the first step in troubleshooting is to check the physical connectivity. If a separate server is used, check that the server is receiving network traffic. If the DHCP service is configured on a router, use the show interfaces command on the router to confirm that the interface is operational. If the interface connected to the host network is down, the port does not pass traffic, including DHCP requests.

Next, verify that the DHCP server is correctly configured and has available IP addresses to lease. After this is confirmed, check for any address conflicts. Address conflicts can occur even if there are available addresses within the DHCP pool. This can happen if a host is statically configured with an address that is also contained in the range of the DHCP pool.

Use the show ip dhcp conflict command to display all address conflicts recorded by the DHCP server. If an address conflict is detected, the address is removed from the pool and not assigned until an administrator resolves the conflict.

If none of these steps diagnoses the problem, test to ensure that the issue is actually with DHCP. Configure a host with a static IP address, subnet mask, and default gateway. If the workstation is unable to reach network resources with a statically configured IP address, the root cause of the problem is not DHCP. At this point, network connectivity troubleshooting is required.

DHCP is a broadcast protocol, which means that the DHCP server must be reachable through a broadcast message. Because routers normally do not forward broadcasts, either the DHCP server must be on the same local network as the hosts or the router must be configured to relay the broadcast messages.

A router can be configured to forward all broadcast packets, including DHCP requests, to a specific server using the ip helper-address command. This command allows a router to change the destination broadcast addresses within a packet to a specified unicast address:

Router(config-if)# ip helper-address x.x.x.x

Once this command is configured, all broadcast packets will be forwarded to the server IP address specified in the command, including DHCP requests.

When a router forwards address requests, it is acting as a DHCP relay agent. If DHCP relay is not operational, no hosts can obtain an IP address. When no hosts can obtain an IP address from a DHCP server that is located on another network, verify that the helper address is configured correctly on the router.

If the hosts on the internal network are assigned private addresses, NAT is required to communicate with the public network. Usually the first indication that there is a NAT problem is that users cannot reach sites located on the Internet. There are three types of address translation: static, dynamic, and PAT. Two common types of configuration errors affect all three translation methods.

Incorrect Designation of Inside and Outside Interfaces
It is critical that the correct interfaces are designated as the inside or outside interface for NAT. In most NAT implementations, the inside interface connects to the local network, which uses private IP address space. The outside interface connects to the public network, usually the ISP. Verify this configuration using the show running-config interface command.

Incorrect Assignment of Interface IP Address or Pool Addresses
In most NAT implementations, the IP address pool and static NAT translation entries must use IP addresses that are on the same local IP network as the outside interface. If not, addresses are translated, but no route to the translated addresses are found. Check the configuration to verify that all the translated addresses are reachable. When the address translation is configured to use the outside interface address in PAT, make sure that the interface address is on the correct network and is configured with the proper subnet mask.

Another common issue is that when dynamic NAT or PAT is enabled, external users are no longer able to connect to internal devices. If external users must be able to reach specific servers on the internal network, be sure that static translations are configured.

If you are certain that NAT is configured correctly, it is important to verify that NAT is operational.

One of the most useful commands when verifying NAT operation is the show ip nat translations command. After viewing the existing translations, clear them using the clear ip nat translation * command. Be aware that clearing all IP translations on a router may disrupt user services. Then use the show ip nat translations command again. If new translations appear, there may be another problem causing the loss of Internet connectivity.

Verify that there is a route to the Internet for the translated addresses. Use traceroute to determine the path the translated packets are taking and verify that the route is correct. Also, if possible, trace the route to a translated address from a remote device on the outside network. This can help isolate the next troubleshooting target. There may be a routing problem on the router where the trace output stops.

Layer 3 Routing Issues

Layer 3 encompasses the addressing of networks and hosts, and the protocols that route packets between networks.

Most networks have a number of different types of routes, including a combination of static, dynamic, and default routes. Problems with routing can cause network failures or adversely affect network performance. These problems can be the result of manual route entry errors, routing protocol configuration and operation errors, or failures at lower layers of the OSI model.

To troubleshoot Layer 3 problems, it is important to understand how routing works, including how each type of route functions and is configured.

The status of a network can change frequently for a variety of reasons, including:
– An interface fails.
– A service provider drops a connection.
– The available bandwidth is overloaded.
– An administrator enters an incorrect configuration.

When there is a change in the network status, routes can be lost, or an incorrect route can be installed into the routing table.

The primary tool to use when troubleshooting Layer 3 routing problems is the show ip route command. This command displays all the routes the router uses to forward traffic. The routing table consists of route entries from the following sources:
– Directly connected networks
– Static routes
– Dynamic routing protocols

Routing protocols choose which routes are preferred based on route metrics. Directly connected networks have a metric of 0, static routes also have a default metric of 0, and dynamic routes have various routing metrics, depending on the routing protocol used.

If there is more than one route to a specific destination network, the route with the lowest administrative distance (AD) is installed into the routing table.

Any time a routing problem is suspected, use the show ip route command to ensure that all the expected routes are installed in the routing table.

Connected Route Problems
Directly connected routes are automatically installed in the routing table when an IP address is configured on an interface, and the interface is enabled using the no shutdown command. If a directly connected route does not appear in the table, use the show interfaces or show ip interface brief command to verify that an address is assigned and that the interface is in an up/up state.

Static and Default Route Problems
When a static or default route does not appear in the routing table, the problem is most likely a configuration error. Static and default routes must use either an exit interface or the IP address of a next hop router. Static routing errors sometimes occur because the next hop address is not in the correct IP address range of any directly connected network. Verify that the configuration statements are correct and that the exit interfaces used by the routes are in an up/up state.

Dynamic Route Problems
There are many different types of problems that can cause dynamic routes to not appear in the routing table. Because dynamic routing protocols exchange route tables with all other routers in the network, a missing route could be caused by a misconfiguration on one or more of the routers on the path to the destination.

Dynamic Routing Errors

Routing table updates usually occur when a new network is configured or an already configured network becomes unreachable.

If directly connected routes appear in the router table, the routing table is accessed and changed only if the directly connected interface changes states. If static or default routes are configured, the routing table changes only if new routes are specified or if the exit interface specified in the static or default route changes states.

Dynamic routing protocols automatically send updates to other routers in the network. If dynamic routing is enabled, a router accesses and changes its own routing table any time a change is reported in an update from a neighboring router.

RIP is a dynamic routing protocol used in small- to medium-sized LANs. When troubleshooting issues specific to RIP, check the versioning and configuration statements.

It is always best to use the same version of the routing protocol on all routers. Although RIPv1 and RIPv2 are compatible, RIPv1 does not support classless routing or variable length subnet masks (VLSM). This can create issues if both RIPv1 and RIPv2 are configured to run on the same network. Additionally, while RIPv2 automatically listens for both RIPv1 and RIPv2 updates from neighbors, RIPv1 does not listen for RIPv2 updates.

Routing problems also occur if there are incorrect or missing network statements. The network statement does two things:
– It enables the routing protocol to send and receive updates on any local interfaces that belong to that network.
– It includes that network in its routing updates to its neighboring routers.

A missing or incorrect network statement results in inaccurate routing updates and can prevent an interface from sending or receiving routing updates.

Many tools exist for troubleshooting dynamic routing issues.

TCP/IP utilities, such as ping and traceroute, are used to verify connectivity. Telnet can be used to verify connectivity and make configuration changes. Cisco IOS show commands display a snapshot of a configuration or the status of a particular component. The Cisco IOS command set also includes various debug commands.

Debug commands are dynamic and provide real-time information on traffic movement and the interaction of protocols. For example, the debug ip rip command displays the exchange of RIP routing updates and packets as they occur.

Debug functions use a significant portion of CPU resources and can slow or stop normal router operations. For this reason, use debug commands to isolate problems, not to monitor normal network operation.

Layer 4 Traffic Filtering Errors

Layer 4, the Transport Layer, is considered a transition between the upper and lower layers of the OSI model. Layer 4 is responsible for transporting data packets and specifies the port number used to reach specific applications. Layer 4 network problems can arise at the edge of the network where security technologies are examining and modifying the traffic. Many problems are caused by firewalls that are configured to deny traffic based on port numbers, even though this traffic should be forwarded.

Layer 4 supports both UDP and TCP traffic. Some applications use TCP, some use UDP, and some use both. When denying traffic based on the port number, it is necessary to specify the transport protocol used. Some engineers are unsure of which transport protocol is used by specific applications and therefore deny the port number for both TCP and UDP traffic. This practice may unexpectedly deny traffic that should be allowed.

Firewalls are also often configured to deny everything except the applications specified in the permit statements. If traffic that should be permitted is not included in the firewall statements, or if a new application is added to the network without a corresponding permission being added to the firewall, filtering problems occur.

A common indication of Layer 4 problems is users reporting that some web services, especially video or audio, are not reachable.

Verify that the ports being permitted and denied by the firewall are the correct ones for the applications.

Troubleshooting Upper Layer Problems

Most of the upper layer protocols provide user services that are typically used for network management, file transfer, distributed file services, terminal emulation, and email. Protocols at these layers are often referred to as TCP/IP Application Layer protocols, because the TCP/IP model Application Layer encompasses the upper three layers of the OSI model.

The most widely known and implemented TCP/IP Application Layer protocols include:
Telnet – Enables users to establish terminal session connections with remote hosts.
HTTP – Supports the exchange of text, graphic images, sound, video, and other multimedia files on the web.
FTP – Performs interactive file transfers between hosts, using TCP.
TFTP – Performs basic interactive file transfers typically between hosts and networking devices, using UDP .
SMTP – Supports basic email message delivery services.
POP3 – Connects to mail servers and downloads email to a client application.
IMAP4 – Enables email clients to retrieve messages and store email on servers.
SNMP – Collects information from managed devices.
NTP – Provides updated time to hosts and network devices.
DNS – Maps IP addresses to the names assigned to hosts.
SSL – Provides encryption and security for HTTP transactions.
SSH – Provides secure remote terminal access to servers and networking devices.

It can be difficult to isolate problems to the upper layers, especially if the client configuration does not reveal any obvious problems. To determine that a network problem is with an upper layer function, start by eliminating basic connectivity as the source of the problem.

Using the “divide and conquer” method of troubleshooting, begin with verifying Layer 3 connectivity.

Step 1. Ping the host default gateway.

Step 2. Verify end-to-end connectivity.

Step 3. Verify the routing configuration.

Step 4. Ensure that NAT is working correctly.

Step 5. Check for firewall filter rules.

If the problem exists on a remote network, end-to-end connectivity cannot be verified because there is no control over all the connections. For this reason, it is possible that even though the configurations on the local devices are correct, there is still a problem with the remote network. Be sure to check with the ISP to ensure that their network connection is up and operational.

If all these steps are completed successfully, and it is verified that the end-to-end connectivity is not the issue, but the end device is still not operating as expected, the problem has been isolated to the upper layers.

Upper layer problems prevent services from being provided to application programs. A problem at the upper layers can result in unreachable or unusable resources, even when the lower layers are functional. It is possible to have full network connectivity, but the application cannot provide data.

Problems with upper layer functions usually affect just a few applications, perhaps even only one. It is not unusual for a help desk technician to get a call from a user who cannot receive email, although all other applications are functioning correctly.

Misconfigured client applications account for the majority of upper layer network problems. When an incorrect email or FTP server is specified, the client cannot find and retrieve information. When more than one application is affected, the upper layer problem may be attributed to a DNS server issue.

To verify that DNS is functioning correctly and can resolve server addresses, use the Windows command nslookup. If DNS is not working as expected, ensure that the correct DNS server address is configured on the host. When hosts receive DNS server information from a DHCP server, verify that the DHCP server has the correct IP address for the DNS server.

If the DNS server is operational and reachable, check for DNS zone configuration errors. Look for a typographical error in an address or name within the files.

The upper layers are responsible for encryption and compression. A mismatch between the way a client encrypts or compresses the data and the way the server interprets it can cause applications to not function or to function poorly.

When a problem occurs on a single host or workstation, it may be a problem with the way the information is being interpreted in the host software. Browser plug-in programs, such as Adobe Reader, often perform upper layer functions. These programs must be kept updated for web pages to display correctly.

Using an incorrect protocol to request data can cause a web page to be unreachable. For example, it may be necessary to specify https:// on the browser address line, rather than http:// to retrieve an SSL-protected web page.

Using Telnet to Check Upper Layer Connectivity

Telnet is an excellent tool to use when troubleshooting problems with upper layer functions. Using Telnet to access the networking devices enables the technician to enter commands on each device as if they were locally attached. In addition, the ability to reach devices using Telnet indicates that the lower layer connectivity exists between the devices.

However, Telnet is an insecure protocol, which means that all data communicated can be captured and read. If there is a possibility that communications can be intercepted by unauthorized users, Secure Shell (SSH) protocol should be used instead. SSH is a more secure method for remote device access.

Most newer versions of the Cisco IOS software contain an SSH server. In some devices, this service is enabled by default. Other devices require the SSH server to be manually enabled.

Cisco IOS devices also include an SSH client that can be used to establish SSH sessions with other devices. Similarly, a remote computer with an SSH client can be used to start a secure CLI session. SSH client software is not provided by default on all computer operating systems. The technician may need to acquire, install, and configure SSH client software on the computer.


Chapter 8 – ISP Responsibility

Filed under: Uncategorized — krystalchisholm @ 10:03 am

ISP Security Services

Any active Internet connection for a computer can make that computer a target for malicious activity. Malware, or malicious software such as a computer virus, worm, or spyware, can arrive in an email or be downloaded from a website. Problems that cause large-scale failures in ISP networks often originate from unsecured desktop systems at the ISP customer locations.

If the ISP is hosting any web or e-commerce sites, the ISP may have confidential files with financial data or bank account information stored on their servers. The ISP is required to maintain the customer data in a secure way.

ISPs play a big role in helping to protect the home and business users that use their services. The security services that they provide also protect the servers that are located at the service provider premise. Service providers are often called upon to help their customers secure their local networks and workstations to reduce the risks of compromise.

There are many actions that can be taken both at the local site and the ISP to secure operating systems, data stored on operating systems, and data transmitted between computer systems.

If an ISP is providing web hosting or email services for a customer, it is important that the ISP protect that information from malicious attack. This protection can be complicated because ISPs often use a single server, or cluster of servers, to maintain data that belongs to more than one customer.

To help prevent attacks on these vulnerabilities, many ISPs provide managed desktop security services for their customers. An important part of the job of an on-site support technician is to implement security best practices on client computers. Some of the security services that an ISP support technician can provide include:

Helping clients to create secure passwords for devices
Securing applications using patch management and software upgrades
Removing unnecessary applications and services that can create vulnerabilities
Ensuring applications and services are available to the users that need them and no one else
Configuring desktop firewalls and virus-checking software
Performing security scans on software and services to determine vulnerabilities that the technician must protect from attack.

Security Practices

It is critical that ISPs have measures in place to protect the information of its customers from malicious attack. Common data security features and procedures include:
– Encrypting data stored on server hard drives
– Using permissions to secure access to files and folders
– Permit or deny access based on the user account or group membership
– Assign different levels of access permission based on the user account or group membership

When assigning permissions to files and folders, a security best practice is to apply permissions based on the “principle of least privilege”. This means giving users access to only those resources that are required for them to be able do their job. It also means giving the appropriate level of permission, for example read-only access or write access.

Authentication, Authorization, and Accounting (AAA) is a three-step process used by network administrators to make it difficult for attackers to gain access to a network.

Authentication requires users to prove their identity using a username and password. Authentication databases are typically stored on servers that use the RADIUS or TACACS protocols.

Authorization gives users rights to access specific resources and perform specific tasks.

Accounting tracks which applications are used and the length of time that they are used.

For example, authentication acknowledges that a user named “student” exists and is able to log on. Authorization services specify that user student can access host server XYZ using Telnet. Accounting tracks that user student accessed host server XYZ using Telnet on a specific day for 15 minutes.

AAA can be used on various types of network connections. AAA requires a database to keep track of user credentials, permissions, and account statistics. Local authentication is the simplest form of AAA and keeps a local database on the gateway router. If an organization has more than a handful of users authenticating with AAA, the organization must use a database on a separate server.

Data Encryption

ISPs must also be concerned with securing data that is transmitted to and from their servers. By default, data sent over the network is unsecured and transmitted in clear text. Unauthorized individuals can intercept unsecured data as it is being transmitted. Capturing data in transit bypasses all file system security that is set on the data. There are methods available to protect against this security issue.

Digital encryption is the process of encrypting all transmitted data between the client and the server. Many of the protocols used to transmit data offer a secure version that uses digital encryption. As a best practice, use the secure version of a protocol whenever the data being exchanged between two computers is confidential.

For example, if a user must submit a username and password to log on to an e-commerce website, a secure protocol is required to protect the username and password information from being captured. Secure protocols are also needed any time a user must submit a credit card or bank account information.

When surfing the Internet and viewing publicly accessible websites, securing the transmitted data is not necessary. Using a secure protocol in this situation can lead to additional computational overhead and slower response time.

There are many network protocols used by applications. Some offer secure versions and some do not:
Web servers – Web servers use HTTP by default, which is not a secure protocol. Using HTTPS, which uses the secure socket layer (SSL) protocol, enables the exchange of data to be performed securely.
Email servers – Email servers use several different protocols, including SMTP, POP3, and IMAP4. When a user logs on to an email server, POP3 and IMAP4 require a username and password for authentication. By default, this information is sent without security and can be captured. POP3 can be secured by using SSL. SMTP and IMAP4 can use either SSL or Transport Layer Security (TLS) as a security protocol.
Telnet servers – Using Telnet to remotely log into a Cisco router or switch creates an unsecure connection. Telnet sends authentication information and any commands a user types across the network in clear text. Use the Secure Shell (SSH) protocol to authenticate and work with the router or switch securely.
FTP servers – FTP is also an unsecure protocol. When logging into an FTP server, authentication information is sent in clear text. FTP can use SSL to securely exchange authentication and data. Some versions of FTP can also use SSH.
File servers – File servers can use many different protocols to exchange data, depending on the computer operating system. In most cases, file server protocols do not offer a secure version.

IP Security (IPSec) is another Network Layer security protocol that can be used to secure any Application Layer protocol used for communication. This includes file server protocols that do not offer any other security protocol version.

Access Control Lists and Port Filtering

Even with the use of AAA and encryption, there are still many different types of attacks that an ISP must protect against. ISPs are especially vulnerable to denial-of-service (DoS) attacks, because the ISP may host sites for many different registered domain names that may or may not require authentication. Currently, there are three key types of DoS attacks.

A standard DoS attack is when a server or service is attacked to prevent legitimate access to that service. Some examples of standard DoS attacks are SYN floods, ping floods, LAND attacks, bandwidth consumption attacks, and buffer overflow attacks.

A distributed denial-of-service (DDoS) attack occurs when multiple computers are used to attack a specific target. The attacker has access to many compromised computer systems, usually on the Internet. Because of this, the attacker can remotely launch the attack. DDoS attacks are usually the same kinds of attacks as standard DoS attacks, except that DDoS attacks are run from many computer systems simultaneously.

A distributed reflected denial-of-service (DRDoS) attack occurs when an attacker sends a spoofed, or mock, request to many computer systems on the Internet, with the source address modified to be the targeted computer system. The computer systems that receive the request respond. When the computer systems respond to the request, all the requests are directed at the target computer system. Because the attack is reflected, it is very difficult to determine the originator of the attack.

ISPs must be able to filter out network traffic, such as DoS attacks, that can be harmful to the operation of their network or servers. Port filtering and access control lists (ACL) can be used to control traffic to servers and networking equipment.

Port Filtering
Port filtering controls the flow of traffic based on a specific TCP or UDP port. Many server operating systems have options to restrict access using port filtering. Port filtering is also used by network routers and switches to help control traffic flow and to secure access to the device.

Access Control Lists
ACLs define traffic that is permitted or denied through the network based on the source and destination IP addresses. ACLs can also permit or deny traffic based on the source and destination ports of the protocol being used. Additonally, ICMP and routing update traffic can be controlled using ACLs. Administrators create ACLs on network devices, such as routers, to control whether or not traffic is forwarded or blocked.

ACLs are only the first line of defense and are not enough to secure a network. ACLs only prevent access to a network; they do not protect the network from all types of malicious attacks.


A firewall is network hardware or software that defines which traffic can come into and go out of sections of the network and how traffic is handled.

ACLs are one of the tools used by firewalls. ACLs control which type of traffic is allowed to pass through the firewall. The direction the traffic is allowed to travel can also be controlled. In a medium-sized network, the amount of traffic and networking protocols needing to be controlled is quite large, and firewall ACLs can become very complicated.

Firewalls use ACLs to control which traffic is passed or blocked. They are constantly evolving as new capabilities are developed and new threats are discovered.

Different firewalls offer different types of features. For example, a dynamic packet filter firewall or stateful firewall keeps track of the actual communication process occurring between the source and destination devices. It does this by using a state table. When a communication stream is approved, only traffic that belongs to one of these communication streams is permitted through the firewall. The Cisco IOS Firewall software is embedded in the Cisco IOS software and allows the user to turn a router into a network layer firewall with dynamic or stateful inspection.

Firewalls are constantly evolving as new capabilities are developed and new threats are discovered. The more functionality embedded in a firewall, the more time it takes for packets to be processed.

Firewalls can provide perimeter security for the entire network and for internal local network segments, such as server farms.

Within an ISP network or a medium-sized business, firewalls are typically implemented in multiple layers. Traffic that comes in from an untrusted network first encounters a packet filter on the border router. Permitted traffic goes through the border router to an internal firewall to route traffic to a demilitarized zone (DMZ). A DMZ is used to store servers that users from the Internet are allowed to access. Only traffic that is permitted access to these servers is permitted into the DMZ. Firewalls also control what kind of traffic is permitted into the protected, local network itself. The traffic that is allowed into the internal network is usually traffic that is being sent due to a specific request by an internal device. For example, if an internal device requests a web page from an external server, the firewall permits the requested web page to enter the internal network.

Some organizations can choose to implement internal firewalls to protect sensitive areas. Internal firewalls are used to restrict access to areas of the network that need to have additional protection. Internal firewalls separate and protect business resources on servers from users inside the organization. Internal firewalls prevent external and internal hackers, as well as unintentional internal attacks and malware.


ISPs also have a responsibility to prevent, when possible, intrusions into their networks and the networks of customers who purchase managed services. There are two tools often utilized by ISPs to accomplish this.

Intrusion Detection System (IDS)
An IDS is a software- or hardware-based solution that passively listens to network traffic. Network traffic does not pass through an IDS device. Instead, the IDS device monitors traffic through a network interface. When the IDS detects malicious traffic, it sends an alert to a preconfigured management station.

Intrusion Prevention System (IPS)
An IPS is an active physical device or software feature. Traffic travels in one interface of the IPS and out the other. The IPS examines the actual data packets that are in the network traffic and works in real time to permit or deny packets that want access into the network

IDS and IPS technologies are deployed as sensors. An IDS or an IPS sensor can be any of the following:
– Router configured with Cisco IOS version IPS
– Appliance (hardware) specifically designed to provide dedicated IDS or IPS services
– Network module installed in an adaptive security appliance (ASA), switch, or router

IDS and IPS sensors respond differently to incidences detected on the network, but both have roles within a network.

IDS solutions are reactive when it comes to detecting intrusions. They detect intrusions based on a signature for network traffic or computer activity. They do not stop the initial traffic from passing through to the destination, but react to the detected activity.

When properly configured, the IDS can block further malicious traffic by actively reconfiguring network devices, such as security appliances or routers, in response to malicious traffic detection. It is important to realize that the original malicious traffic has already passed through the network to the intended destination and cannot be blocked. Only subsequent traffic is blocked. In this regard, IDS devices cannot prevent some intrusions from being successful.

IDS solutions are often used on the untrusted perimeter of a network, outside of the firewall. Here the IDS can analyze the type of traffic that is hitting the firewall and determine how attacks are executed. The firewall can be used to block most malicious traffic. An IDS can also be placed inside the firewall to detect firewall misconfigurations. When the IDS sensor is placed here, any alarms that go off indicate that malicious traffic has been allowed through the firewall. These alarms mean that the firewall has not been configured correctly.

Unlike IDS solutions, which are reactive, IPS solutions are proactive. They block all suspicious activity in real time. An IPS is able to examine almost the entire data packet from Layer 2 to Layer 7 of the OSI model. When the IPS detects malicious traffic, it blocks the malicious traffic immediately. The IPS then sends an alert to a management station about the intrusion. The original and subsequent malicious traffic is blocked as the IPS proactively prevents attacks.

An IPS is an intrusion detection appliance, not software. The IPS is most often placed inside the firewall. This is because it can examine most of the data packet and, therefore, be used to protect server applications if malicious traffic is being sent. The firewall typically does not examine the entire data packet, whereas the IPS does. The firewall drops most of the packets that are not allowed, but may still allow some malicious packets through. The IPS has a smaller number of packets to examine, so it can examine the entire packet. This allows the IPS to immediately stop new attacks that the firewall was not originally configured to deny. IPS can also stop attacks that the firewall is unable to deny based on limitations of the firewall.

Wireless Security

Some ISPs offer services to create wireless hot spots for customers to log on to wireless local-area networks (WLANs). A wireless network is easy to implement, but is vulnerable when not properly configured. Because the wireless signal travels through walls, it can be accessed outside the business premises. A wireless network can be secured by changing the default settings, enabling authentication, or enabling MAC address filtering.

Changing Default Settings
The default values for the SSID, usernames, and passwords on a wireless access point should be changed. Additionally, broadcasting of the SSID should be disabled.

Enabling Authentication
Authentication is the process of permitting entry to a network based on a set of credentials. It is used to verify that the device attempting to connect to the network is trusted. There are three types of authentication methods that can be used:
– Open authentication – Any and all clients are able to have access regardless of who they are. Open authentication is most often used on public wireless networks.
Pre-shared key (PSK) – Requires a matching, preconfigured key on both the server and the client. When connecting, the access point sends a random string of bytes to the client. The client accepts the string, encrypts it (or scrambles it) based on the key, and sends it back to the access point. The access point gets the encrypted string and uses its key to decrypt (or unscramble) it. If they match, authentication is successful.
– Extensible Authentication Protocol (EAP) – Provides mutual, or two-way, authentication and user authentication. When EAP software is installed on the client, the client communicates with a backend authentication server, such as RADIUS.

Enabling MAC Address Filtering
MAC address filtering prevents unwanted computers from connecting to a network by restricting MAC addresses. It is possible, however, to clone a MAC address. Therefore, other security measures should be implemented along with MAC address filtering.

It is important to set encryption on transmitted packets sent across a wireless network. There are three major encryption types for wireless networks:
WEP – Wired Equivalent Privacy (WEP) provides data security by encrypting data that is sent between wireless nodes. WEP uses a 64, 128, or 256 bit pre-shared hexadecimal key to encrypt the data. A major weakness of WEP is its use of static encryption keys. The same key is used by every device to encrypt every packet transmitted. There are many WEP cracking tools available on the Internet. WEP should be used only with older equipment that does not support newer wireless security protocols.
WPA – Wifi Protected Access (WPA) is a newer wireless encryption protocol that uses an improved encryption algorithm called Temporal Key Integrity Protocol (TKIP). TKIP generates a unique key for each client and rotates the security keys at a configurable interval. WPA provides a mechanism for mutual authentication. Because both the client and the access point have the key, it is never transmitted.
WPA2 – WPA2 is a new, improved version of WPA. WPA2 uses the more secure Advanced Encryption Standard (AES) technology.

Host Security

Regardless of the layers of defense that exist on the network, all servers are still susceptible to attack if they are not properly secured. ISP servers are especially vulnerable because they are generally accessible from the Internet. New vulnerabilities for servers are discovered every day, so it is critical for an ISP to protect its servers from known and unknown vulnerabilities whenever possible. One way they accomplish this is by using host-based firewalls.

A host-based firewall is software that runs directly on a host operating system. It protects the host from malicious attacks that might have made it through all other layers of defense. Host-based firewalls control inbound and outbound network traffic. These firewalls allow filtering based on a computer address and port, therefore offering additional protection over regular port filtering.

Host-based firewalls typically come with predefined rules that block all incoming network traffic. Exceptions are added to the firewall rule set to permit the correct mixture of inbound and outbound network traffic. When enabling host-based firewalls, it is important to balance the need to allow the network resources required to complete job tasks, with the need to prevent applications from being left vulnerable to malicious attacks. Many server operating systems are preconfigured with a simple host-based firewall with limited options. More advanced third-party packages are also available.

ISPs use host-based firewalls to restrict access to the specific services a server offers. By using a host-based firewall, the ISP protects their servers and the data of their customers by blocking access to the extraneous ports that are available.

ISP servers that utilize host-based firewalls are protected from a variety of different types of attacks and vulnerabilities.

Known Attacks
Host-based firewalls recognize malicious activity based on updatable signatures or patterns. They detect a known attack and block traffic on the port used by the attack.

Exploitable Services
Host-based firewalls protect exploitable services running on servers by preventing access to the ports that the service is using. Some host-based firewalls can also inspect the contents of a packet to see if it contains malicious code. Web and email servers are common targets for service exploits, and can be protected if the host-based firewall is capable of performing packet inspection.

Worms and Viruses
Worms and viruses propagate by exploiting vulnerabilities in services and other weaknesses in operating systems. Host-based firewalls prevent this malware from gaining access to servers. They can also help prevent the spread of worms and viruses by controlling outbound traffic originating from a server.

Back Doors and Trojans
Back doors and Trojan Horses allow hackers to remotely gain access to servers on a network. The software typically works by sending a message to let the hacker know of a successful infection. It then provides a service that the hacker can use to gain access to the system. Host-based firewalls can prevent a Trojan from sending a message by limiting outbound network access. It can also prevent the attacker from connecting to any services.

In addition to host-based firewalls, anti-X software can be installed as a more comprehensive security measure. Anti-X software protects computer systems from viruses, worms, spyware, malware, phishing, and even spam. Many ISPs offer customers anti-X software as part of their comprehensive security services. Not all anti-X software protects against the same threats. The ISP should constantly review which threats the anti-X software actually protects against and make recommendations based on a threat analysis of the company.

Many anti-X software packages allow for remote management. This includes a notification system that can alert the administrator or support technician about an infection via email or pager. Immediate notification to the proper individual can drastically reduce the impact of the infection. Using anti-X software does not diminish the number of threats to the network but reduces the risk of being infected.

Occasionally infections and attacks still occur and can be very destructive. It is important to have an incident management process to track all incidences and the corresponding resolutions to help prevent the infection from reoccurring. Incident management is required by ISPs that manage and maintain customer data, because the ISP has committed to the protection and the integrity of the data they host for their customers. For example, if the ISP network was the target of a hacker and, as a result, thousands of credit card numbers that were stored in a database that the ISP manages were stolen, the customer would need to be notified so that they could notify the card holders.

Service Level Agreements

An ISP and a user usually have a contract known as a service level agreement (SLA). It documents the expectations and obligations of both parties. An SLA typically includes the following parts:
– Service description
– Costs
– Tracking and reporting
– Problem management
– Security
– Termination
– Penalties for service outages
– Availability, performance, and reliability

The SLA is an important document that clearly outlines the management, monitoring, and maintenance of a network.

Monitoring Network Link Performance

The ISP is responsible for monitoring and checking device connectivity. This responsibility includes any equipment that belongs to the ISP and equipment at the customer end that the ISP agreed to monitor in the SLA. Monitoring and configuration can be performed either out-of-band with a direct console connection, or in-band using a network connection.

Out-of-band management is useful in initial configurations if the device is not accessible via the network, or if a visual inspection of the device is necessary.

Most ISPs are not able to visually inspect or have physical access to all devices. An in-band management tool allows for easier administration because the technician does not require a physical connection. For this reason, in-band management is preferred over out-of-band management for managing servers and networking devices that are accessible on the network. Additionally, conventional in-band tools can provide more management functionality than may be possible with out-of-band management, such as an overall view of the network design. Traditional in-band management protocols include Telnet, SSH, HTTP, and Simple Network Management Protocol (SNMP).

There are many embedded tools, commercial tools, and shareware tools available that use these management protocols. For example, HTTP access is through a web browser. Some applications, such as Cisco SDM, use this access for in-band management.

Device Management Using In-Band Tools

After a new network device is installed at the customer premise, it must be monitored from the remote ISP location. There are times that minor configuration changes need to be made without the physical presence of a technician at the customer site.

A Telnet client can be used over an IP network connection to connect to a device in-band for the purpose of monitoring and administering it. A connection using Telnet is called a Virtual Terminal (VTY) session or connection. Telnet is a client/server protocol. The connecting device runs the Telnet client. To support Telnet client connections, the connected device, or server, runs a service called a Telnet daemon.

Most operating systems include an Application Layer Telnet client. On a Microsoft Windows PC, Telnet can be run from the command prompt. Other common terminal emulation applications that run as Telnet clients are HyperTerminal, Minicom, and TeraTerm. Devices such as routers run both the Telnet client and the Telnet daemon, and can act as either the client or server.

After a Telnet connection is established, users can perform any authorized function on the server, just as if they were using a command line session on the server itself. If authorized, users can start and stop processes, configure the device, and even shut down the system.

A Telnet session can be initiated using the router CLI with the telnet command followed by the IP address or domain name. A Telnet client can connect to multiple servers simultaneously. On a Cisco router, the keystroke sequence Ctrl-Shift-6 X to toggles between Telnet sessions. Additionally, a Telnet server can support multiple client connections. On a router acting as a server, the show sessions command displays all client connections.

While the Telnet protocol supports user authentication, it does not support the transport of encrypted data. All data exchanged during a Telnet session is transported as plain text across the network. This means that the data can be intercepted and easily understood, including the username and password used to authenticate the device.

If security is a concern, the Secure Shell (SSH) protocol offers an alternate and secure method for server access. SSH provides secure remote login and other network services. It also provides stronger authentication than Telnet and supports the transport of session data using encryption. As a best practice, network professionals should always use SSH in place of Telnet whenever possible.

There are two versions of the SSH server service. Which SSH version is supported depends on the Cisco IOS image loaded on the device. There are many different SSH client software packages available for PCs. An SSH client must support the SSH version configured on the server.

Using SNMP and Syslog

SNMP is a network management protocol that enables administrators to gather data about the network and corresponding devices. SNMP management system software is available in tools such as CiscoWorks. There are free versions of CiscoWorks available for download on the Internet. SNMP management agent software is often embedded in operating systems on servers, routers, and switches.

SNMP is made up of four main components:
Management station – Computer with the SNMP management application loaded that is used by the administrator to monitor and configure the network.
Management agent – Software installed on a device managed by SNMP.
Management Information Base (MIB) – Database that a device keeps about itself concerning network performance parameters.
Network management protocol – Communication protocol used between the management station and the management agent.

The management station contains the SNMP management applications that the administrator uses to configure devices on the network. It also stores data about those devices. The management station collects information by polling the devices. A poll occurs when the management station requests specific information from an agent.

The agent reports to the management station by responding to the polls. When the management station polls an agent, the agent calls on statistics that have accumulated in the MIB.

Agents can also be configured with traps. A trap is an alarm-triggering event. Certain areas of the agent are configured with thresholds, or maximums, that must be maintained, such as the amount of traffic that can access a specific port. If the threshold is exceeded, the agent sends an alert message to the management station. Traps free the management station from continuously polling network devices.

Management stations and managed devices are identified by a community ID, called a community string. The community string on the SMNP agent must match the community string on the SMNP management station. When an agent is required to send information to a management station due to a poll or trap event, it will first verify the management station using the community string.

Storing device logs and reviewing them periodically is an important part of network monitoring. Syslog is the standard for logging system events. Like SNMP, syslog is an Application Layer protocol that enables devices to send information to a syslog daemon that is installed and running on a management station.

A syslog system is composed of syslog servers and syslog clients. These servers accept and process log messages from syslog clients. A syslog client is a monitored device that generates and forwards log messages to syslog servers.

Log messages normally consist of a ID, type of message, a time stamp (date, time), which device has sent the message, and the message text. Depending on which network equipment is sending the syslog messages, it can contain more items than those listed.

Backup Media

Network management and monitoring software helps ISPs and businesses identify and correct network issues. This software can also help to correct the causes of network failures, such as those caused by malware and malicious activity, network functionality, and failed devices.

Regardless of the cause of failure, an ISP that hosts websites or email for customers must protect the web and email content from being lost. Losing the data stored on a website could mean hundreds, or even thousands, of hours recreating the content, not to mention the lost business that results from the downtime while the content is being restored.

Losing email messages that were stored on the ISP email server could potentially be devastating for a business that relies on the data within the emails. Some businesses are legally required to maintain records of all email correspondence, so losing email data is not acceptable.

Data backup is essential. The job of an IT professional is to reduce the risks of data loss and provide mechanisms for quick recovery of any data that is lost.

When an ISP needs to back up its data, the cost of a backup solution and its effectiveness must be balanced. The choice of backup media can be complex because there are many factors that affect the choice.

Some of the factors include:
– Amount of data
– Cost of media
– Performance of media
– Reliability of media
– Ease of offsite storage

There are many types of backup media available, including tapes, optical discs, hard disks, and solid state devices.

Tape remains one of the most common types of backup media available. Tapes have large capacities and remain the most cost-effective media on the market. For data volumes in excess of a single tape, autoloaders and libraries can swap tapes during the backup procedure, allowing the data to be stored on as many tapes as required. These devices can be expensive and are not typically found in small to medium-sized businesses. However, depending on the volume of data, there may be no alternative other than an autoloader or library.

Tape Media
Tape media is prone to failure, and tape drives require regular cleaning to maintain functionality. Tapes also have a high failure rate because they wear out through use. Tapes should only be used for a fixed amount of time before removing them from circulation. Some of the different types of tapes are:
– Digital data storage (DDS)
– Digital audio tape (DAT)
– Digital linear tape (DLT)
– Linear tape-open (LTO)

Each type has different capacities and performance characteristics.

Optical Media Discs
Optical media is a common choice for smaller amounts of data. CDs have a storage capacity of 700 MB, DVDs can support up to 8.5 GB on a single-sided dual layer disc, and HD-DVD and Blu-Ray discs can have capacities in excess of 25 GB per disc. ISPs may use optical media for transferring web content data to their customers. Customers may also use this media to transfer website content to the ISP web hosting site. Optical media can easily be accessed by any computer system with a CD or DVD drive.

Hard Disks
Hard disk-based backup systems are becoming more and more popular because of the low cost of high-capacity drives. However, hard disks make offsite storage difficult. Large disk arrays such as direct attached storage (DAS), network attached storage (NAS), and storage area networks (SANs) are not transportable.

Many implementations of hard disk-based backup systems work in conjunction with tape backup systems for offsite storage. Using both hard disks and tapes in a tiered backup solution provides a quick restore time with the data available locally on the hard disks combined with a long-term archival solution.

Solid State Storage Devices
Solid state storage refers to all nonvolatile storage media that does not have any moving parts. Examples of solid state media range from small postage-stamp-sized drives holding 1 GB of data, to router-sized packages capable of storing 1000 GB (1TB) of data.

Solid state devices are ideal when fast storage and retrieval of data is important. Applications for solid state data storage systems include database acceleration, high-definition video access and editing, data retrieval, and SANS. High-capacity solid state storage devices can be extremely expensive, but as the technology matures, the prices will come down.

Methods of File Backup

After backup media is chosen, a backup method must be selected.

A normal, or full, backup copies all selected files, in their entirety. Each file is then marked as having been backed up. With normal backups, only the most recent backup is required to restore files. This speeds up and simplifies the restore process. However, because all data is backed up, a full backup takes the most amount of time.

A differential backup copies only the files that have been changed since the last full backup. With differential backups, a full backup on the first day of the backup cycle is necessary. Only the files that are created or changed since the time of the last full backup are then saved. The differential backup process continues until another full backup is run. This reduces the amount of time required to perform the backup. When it is time to restore data, the last normal backup is restored and the latest differential backup restores all changed files since the last full backup.

An incremental backup differs from a differential backup on one important point. Whereas a differential backup saves files that were changed since the last full backup, an incremental backup only saves files that were created or changed since the last incremental backup. This means that if an incremental backup is run every day, the backup media would only contain files created or changed on that day. Incremental backups are the quickest backup. However, they take the longest time to restore because the last normal backup and every incremental backup since the last full backup must be restored.

Backup systems require regular maintenance to keep them running properly. There are measures that help to ensure that backups are successful:
Swap media – Many backup scenarios require daily swapping of media to maintain a history of backed up data. Data loss could occur if the tape or disk is not swapped daily. Because swapping the tapes is a manual task, it is prone to failure. Users need to use a notification method, such as calendar or task scheduling.
Review backup logs – Virtually all backup software produces logs. These logs report on the success of the backup or specify where it failed. Regular monitoring of backup logs allows for quick identification of any backup issues that require attention.
Perform trial restores – Even if a backup logs shows that the backup was successful, there could be other problems not indicated in the log. Periodically perform a trial restore of data to verify that the backup data is usable and that the restore procedure works.
Perform drive maintenance – Many backup systems require special hardware to perform backups. Tape backup systems use a tape backup drive to read and write to the tapes. Tape drives can become dirty from use and can lead to mechanical failure. Perform routine cleaning of the tape drive using designated cleaning tapes. Hard drive-based backup systems can benefit from an occasional defragmentation to improve the overall performance of the system.

Cisco IOS Software Backup and Recovery

n addition to backing up server files, it is also necessary for the ISP to protect configurations and the Cisco IOS software used on networking devices owned by the ISP. The Cisco networking device software and configuration files can be saved to a network server using TFTP and variations of the copy command. The command to save the IOS file is very similar to the command to backup and save a running configuration file.

To back up Cisco IOS software, there are three basic steps:
Step 1. Ping the TFTP server where the file should be saved. This verifies connectivity to the TFTP server. Use the ping command.
Step 2. On the router, verify the IOS image in flash. Use the show flash command to view the filename of the IOS image and file size. Confirm that the TFTP server has enough disk space to store the file.
Step 3. Copy the IOS image to the TFTP server using the command:
Router# copy flash tftp

When using the copy command, the router will prompt the user for the source filename, the IP address of the TFTP server, and the destination filename.

Images stored on the TFTP server can be used to restore or upgrade the Cisco IOS software on routers and switches in a network.

The steps to upgrade an IOS image file on a router are similar to the steps used to backup the file to the TFTP server. Be sure to use the show flash command to verify the bytes available in flash and confirm that there is enough room for the IOS file before starting the upgrade or restore.

To upgrade the Cisco IOS software, use the command:
copy tftp: flash:

When upgrading, the router will prompt the user to enter the IP address of the TFTP server followed by the filename of the image on the server that should be used. The router may prompt the user to erase the flash memory if there is not sufficient memory available for both the old and the new images. As the image is erased from flash, a series of “e”s appears to indicate the erase process. When the new image is loaded, it is verified, and the networking device is ready to be reloaded with the new Cisco IOS image.

If the IOS image is lost and must be restored, a separate process, using the ROMmon mode is required.

If the router is set to boot up from flash, but the Cisco IOS image in flash is erased, corrupted, or inaccessible because of lack of memory, the image may need to be restored. The quickest way to restore a Cisco IOS image to the router is by using TFTP in ROM monitor (ROMmon) mode.

The ROMmon TFTP transfer works on a specified LAN port, and defaults to the first available LAN interface. To use TFTP in ROMmon mode, the user must first set a few environmental variables, including the IP address, and then use the tftpdnld command to restore the image.

To set a ROMmon environment variable, type the variable name, an equal sign (=), and the value for the variable. For example, to set the IP address to, type IP_ADDRESS=

The required environment variables are:
IP_ADDRESS – IP address on the LAN interface
IP_SUBNET_MASK – Subnet mask for the LAN interface
DEFAULT_GATEWAY – Default gateway for the LAN interface
TFTP_SERVER – IP address of the TFTP server
TFTP_FILE – Cisco IOS filename on the server

Use the set command to view and verify the ROMmon environment variables.

After the variables are set, the tftpdnld command is entered. As each datagram of the Cisco IOS file is received, an exclamation point (!) is displayed. As the Cisco IOS file is copied, the existing flash is erased. This includes all files that may be present in flash memory, not just the current IOS file. For this reason, it is important to back up these files to a TFTP server for safekeeping, in the event that it becomes necessary to restore the IOS image.

When the ROMmon prompt appears (rommon 1>), the router can be restarted using the reset command or typing i. The router should now boot from the new Cisco IOS image in flash.

Disaster Recovery Plan

Data backup is an important part of any disaster recovery plan. A disaster recovery plan is a comprehensive document that describes how to restore operation quickly and keep a business running during or after a disaster occurs. The objective of the disaster recovery plan is to ensure that the business can adapt to the physical and social changes that a disaster causes. A disaster can include anything from natural disasters that affect the network structure to malicious attacks on the network itself.

The disaster recovery plan can include information such as offsite locations where services can be moved, information on switching out network devices and servers, and backup connectivity options. It is important when building a disaster recovery plan to fully understand the services that are critical to maintaining operation. Services that might need to be available during a disaster include:
– Databases
– Application servers
– System management servers
– Web
– Data stores
– Directory

When designing a disaster recovery plan, it is important to understand the needs of the organization. It is also important to gain the support necessary for a disaster recovery plan. There are several steps to accomplish designing an effective recovery plan.

Vulnerability assessment – Assess how vulnerable the critical business processes and associated applications are to common disasters.
Risk assessment – Analyze the risk of a disaster occurring and the associated effects and costs to the business. Part of a risk assessment is creating a list of the top-ten potential disasters and the effects, including the scenario of the business being completely destroyed.
Management awareness – Use the information gathered on vulnerability and risks to get senior management approval on the disaster recovery project. Maintaining equipment and locations in the event of a possible disaster recovery could be expensive. Senior management must understand the possible effect of any disaster situation.
Planning group – Establish a planning group to manage the development and implementation of the disaster recovery strategy and plan. When a disaster occurs, be it small or large scale, it is important that individuals understand their roles and responsibilities.
Prioritize – Assign a priority for each disaster scenario, such as mission critical, important, or minor, for the business network, applications, and systems.

The disaster recovery planning process should first engage the top managers, and then eventually include all personnel that work with critical business processes. Everyone must be involved and support the plan for it to be successful.

fter the services and applications that are most critical to a business are identified, that information should be used to create a disaster recovery plan. There are five major phases to creating and implementing a disaster recovery plan:

Phase 1 – Network Design Recovery Strategy

Analyze the network design. Some aspects of the network design that should be included in the disaster recovery are:
– Is the network designed to survive a major disaster? Are there backup connectivity options and is there redundancy in the network design?
– Availability of offsite servers that can support applications such as email and database services.
– Availability of backup routers, switches, and other network devices should they fail.
– Location of services and resources that the network needs. Are they spread over a wide geography?

Phase 2 – Inventory and Documentation

Create an inventory of all locations, devices, vendors, used services, and contact names. Verify cost estimates that are created in the risk assessment step.

Phase 3 – Verification

Create a verification process to prove that the disaster recover strategy works. Practice disaster recovery exercises to ensure that the plan is up to date and workable.

Phase 4 – Approval and Implementation

Obtain senior management approval and develop a budget to implement the disaster recovery plan.

Phase 5 – Review

After the disaster recovery plan has been implemented for a year, review the plan.

November 24, 2010

Chapter 7 – ISP Services

Filed under: Uncategorized — krystalchisholm @ 10:25 am

Customer Requirements

After the connection is made to the ISP, the business or customer must decide which services they need from the ISP.

ISPs serve several markets. Individuals in homes make up the consumer market. Large, multinational companies make up the enterprise market. In between are smaller markets, such as small- to medium-sized businesses, or larger nonprofit organizations. Each of these customers have different service requirements.

Escalating customer expectations and increasingly competitive markets are forcing ISPs to offer new services. These services enable the ISPs to increase revenue and to differentiate themselves from their competitors.

Email, web hosting, media streaming, IP telephony, and file transfer are important services that ISPs can provide to all customers. These services are critical for the ISP consumer market and for the small- to medium-sized business that does not have the expertise to maintain their own services.

Many organizations, both large and small, find it expensive to keep up with new technologies, or they simply prefer to devote resources to other parts of the business. ISPs offer managed services that enable these organizations to have access to the leading network technologies and applications without having to make large investments in equipment and support.

When a company subscribes to a managed service, the service provider manages the network equipment and applications according to the terms of a service level agreement (SLA). Some managed services are also hosted, meaning that the service provider hosts the applications in its facility instead of at the customer site.

The following are three scenarios that describe different ISP customer relationships:
– Scenario 1 – The customer owns and manages all their own network equipment and services. These customers only need reliable Internet connectivity from the ISP.
– Scenario 2 – The ISP provides Internet connectivity. The ISP also owns and manages the network connecting equipment installed at the customer site. ISP responsibilities include setting up, maintaining, and administering the equipment for the customer. The customer is responsible for monitoring the status of the network and the applications, and receives regular reports on the performance of the network.
– Scenario 3 – The customer owns the network equipment, but the applications that the business relies on are hosted by the ISP. The actual servers that run the applications are located at the ISP facility. These servers may be owned by the customer or the ISP, although the ISP maintains both the servers and the applications. Servers are normally kept in server farms in the ISP network operations center (NOC), and are connected to the ISP network with a high-speed switch.

Reliability & Availability

Creating new services can be challenging. Not only must ISPs understand what their customers want, but they must have the ability and the resources to provide those services. As business and Internet applications become more complex, an increasing number of ISP customers rely on the services provided or managed by the ISP.

ISPs provide services to customers for a fee and guarantee a level of service in the SLA. To meet customer expectations, the service offerings have to be reliable and available.
Reliability can be measured in two ways: mean time between failure (MTBF) and mean time to repair MTTR. Equipment manufacturers specify MTBF based on tests they perform as part of manufacturing. The measure of equipment robustness is fault tolerance. The longer the MTBF, the greater the fault tolerance. MTTR is established by warranty or service agreements.

When there is an equipment failure, and the network or service becomes unavailable, it impacts the ability of the ISP to meet the terms of the SLA. To prevent this, an ISP may purchase expensive service agreements for critical hardware to ensure rapid manufacturer or vendor response. An ISP may also choose to purchase redundant hardware and keep spare parts on site.
Availability is normally measured in the percentage of time that a resource is accessible. A perfect availability percentage is 100%, meaning that the system is never down or unreachable. Traditionally, telephone services are expected to be available 99.999% of the time. This is called the five-9s standard of availability. With this standard, only a very small percentage (0.001%) of downtime is acceptable. As ISPs offer more critical business services, such as IP telephony or high-volume retail sale transactions, ISPs must meet the higher expectations of their customers. ISPs ensure accessibility by doubling up on network devices and servers using technologies designed for high availability. In redundant configurations, if one device fails, the other one can take over the functions automatically.

Review of TCP/IP Protocols

Today, ISP customers are using mobile phones as televisions, PCs as telephones, and televisions as interactive gaming stations with many different entertainment options. As network services become more advanced, ISPs must accommodate these customer preferences. The development of converged IP networks enables all of these services to be delivered over a common network.

To provide support for the multiple end-user applications that rely on TCP/IP for delivery, it is important for the ISP support personnel to be familiar with the operation of the TCP/IP protocols.

ISP servers need to be able to support multiple applications for many different customers. For this support, they must use functions provided by the two TCP/IP transport protocols, TCP and UDP. Common hosted applications, like web serving and email accounts, also depend on underlying TCP/IP protocols to ensure their reliable delivery. In addition, all IP services rely on domain name servers, hosted by the ISPs, to provide the link between the IP addressing structure and the URLs that customers use to access them.

Clients and servers use specific protocols and standards when exchanging information. The TCP/IP protocols can be represented using a four-layer model. Many of the services provided to ISP customers depend on protocols that reside at the Application and Transport layers of the TCP/IP model.

Application Layer Protocols
Application Layer protocols specify the format and control the information necessary for many of the common Internet communication functions. Among these protocols are:
– Domain Name System (DNS) – Resolves Internet names to IP addresses.
– HyperText Transfer Protocol (HTTP) -Transfers files that make up the web pages of the World Wide Web.
– Simple Mail Transfer Protocol (SMTP) – Transfers mail messages and attachments.
– Telnet – Terminal emulation protocol that provides remote access to servers and networking devices.
– File Transfer Protocol (FTP) – Transfers files between systems interactively.

Transport Layer Protocols
Different types of data can have unique requirements. For some applications, communication segments must arrive in a specific sequence to be processed successfully. In other instances, all the data must be received for any of it to be of use. Sometimes, an application can tolerate the loss of a small amount of data during transmission over the network.

In today’s converged networks, applications with very different transport needs may be communicating on the same network. Different Transport Layer protocols have different rules to enable devices to handle these diverse data requirements.

Additionally, the lower layers are not aware that there are multiple applications sending data on the network. Their responsibility is to get the data to the device. It is the job of the Transport Layer to deliver the data to the appropriate application.

OSI vs TCP/IPThe two primary Transport Layer protocols are TCP and UDP.

The TCP/IP model and the OSI model have similarities and differences.

– Use of layers to visualize the interaction of protocols and services
– Comparable Transport and Network layers
– Used in the networking field when referring to protocol interaction

– OSI model breaks the function of the TCP/IP Application Layer into distinct layers. The upper three layers of the OSI model specify the same functionality as the Application Layer of the TCP/IP model.
– The TCP/IP suite does not specify protocols for the physical network interconnection. The two lower layers of the OSI model are concerned with access to the physical network and the delivery of bits between hosts on a local network.

The TCP/IP model is based on actual developed protocols and standards, whereas the OSI model is a theoretical guide for how protocols interact.

Transport Layer Protocols

Different applications have different transport needs. There are two protocols at the Transport Layer: TCP and UDP.

TCP is a reliable, guaranteed-delivery protocol. TCP specifies the methods hosts use to acknowledge the receipt of packets, and requires the source host to resend packets that are not acknowledged. TCP also governs the exchange of messages between the source and destination hosts to create a communication session. TCP is often compared to a pipeline, or a persistent connection, between hosts. Because of this, TCP is referred to as a connection-oriented protocol.

TCP requires overhead, which includes extra bandwidth and increased processing, to keep track of the individual conversations between the source and destination hosts and to process acknowledgements and retransmissions. In some cases, the delays caused by this overhead cannot be tolerated by the application. These applications are better suited for UDP.

UDP is a very simple, connectionless protocol. It provides low overhead data delivery. UDP is considered a “best effort” Transport Layer protocol because it does not provide error checking, guaranteed data delivery, or flow control. Because UDP is a “best effort” protocol, UDP datagrams may arrive at the destination out of order, or may even be lost all together. Applications that use UDP can tolerate small amounts of missing data. An example of a UDP application is Internet radio. If a piece of data is not delivered, there may only be a minor effect on the quality of the broadcast.

Applications, such as databases, web pages, and email, need to have all data arrive at the destination in its original condition, for the data to be useful. Any missing data can cause the messages to be corrupt or unreadable. These applications are designed to use a Transport Layer protocol that implements reliability. The additional network overhead required to provide this reliability is considered a reasonable cost for successful communication.

The Transport Layer protocol is determined by the type of application data being sent. For example, an email message requires acknowledged delivery and therefore would use TCP. An email client, using SMTP, sends an email message as a stream of bytes to the Transport Layer. At the Transport Layer, the TCP functionality divides the stream into segments.

Within each segment, TCP identifies each byte, or octet, with a sequence number. These segments are passed to the Internet Layer, which places each segment in a packet for transmission. This process is known as encapsulation. At the destination, the process is reversed, and the packets are de-encapsulated. The enclosed segments are sent through the TCP process, which converts the segments back to a stream of bytes to be passed to the email server application.

Before a TCP session can be used, the source and destination hosts exchange messages to set up the connection over which data segments can be sent. The two hosts use a three step process to set up the connection.

In the first step, the source host sends a type of message, called a Synchronization Message, or SYN, to begin the TCP session establishment process. The message serves two purposes:
– It indicates the intention of the source host to establish a connection with the destination host over which to send the data.
– It synchronizes the TCP sequence numbers between the two hosts, so that each host can keep track of the segments sent and received during the conversation.

For the second step, the destination host replies to the SYN message with a synchronization acknowledgement, or SYN-ACK, message.

In the last step, the sending host receives the SYN-ACK and it sends an ACK message back to complete the connection setup. Data segments can now be reliably sent.

This SYN, SYN-ACK, ACK activity between the TCP processes on the two hosts is called a three-way handshake.

When a host sends message segments to a destination host using TCP, the TCP process on the source host starts a timer. The timer allows sufficient time for the message to reach the destination host and for an acknowledgement to be returned. If the source host does not receive an acknowledgement from the destination within the allotted time, the timer expires, and the source assumes the message is lost. The portion of the message that was not acknowledged is then re-sent.

In addition to acknowledgement and retransmission, TCP also specifies how messages are reassembled at the destination host. Each TCP segment contains a sequence number. At the destination host, the TCP process stores received segments in a buffer. By evaluating the segment sequence numbers, the TCP process can confirm that there are no gaps in the received data. When data is received out of order, TCP can also reorder the segments as necessary.

Differences Between TCP & UDP

UDP is a very simple protocol. Because it is not connection-oriented and does not provide the sophisticated retransmission, sequencing, and flow control mechanisms of TCP, UDP has a much lower overhead.

UDP is often referred to as an unreliable delivery protocol, because there is no guarantee that a message has been received by the destination host. This does not mean that applications that use UDP are unreliable. It simply means that these functions are not provided by the Transport Layer protocol and must be implemented elsewhere if required.

Although the total amount of UDP traffic found on a typical network is often relatively low, Application Layer protocols that do use UDP include:
– Domain Name System (DNS)
– Simple Network Management Protocol (SNMP)
– Dynamic Host Configuration Protocol (DHCP)
– RIP routing protocol
– Trivial File Transfer Protocol (TFTP)
– Online games

The main differences between TCP and UDP are the specific functions that each protocol implements and the amount of overhead incurred. Viewing the headers of both protocols is an easy way to see the differences between them.

Each TCP segment has 20 bytes of overhead in the header that encapsulates the Application Layer data. This overhead is incurred because of the error-checking mechanisms supported by TCP.

The pieces of communication in UDP are called datagrams. These datagrams are sent as “best effort” and, therefore, only require 8 bytes of overhead.

Supporting Multiple Services

The task of managing multiple simultaneous communication processes is done at the Transport Layer. The TCP and UDP services keep track of the various applications that are communicating over the network. To differentiate the segments and datagrams for each application, both TCP and UDP have header fields that can uniquely identify these applications for data communications purposes.

A source port and destination port are located in the header of each segment or datagram. Port numbers are assigned in various ways, depending on whether the message is a request or a response. When a client application sends a request to a server application, the destination port contained in the header is the port number that is assigned to the application running on the server. For example, when a web browser application makes a request to a web server, the browser uses TCP and port number 80. This is because TCP port 80 is the default port assigned to web-serving applications. Many common applications have default port assignments. Email servers that are using SMTP are usually assigned to TCP port 25.

As segments are received for a specific port, TCP or UDP places the incoming segments in the appropriate queue. For instance, if the application request is for HTTP, the TCP process running on a web server places incoming segments in the web server queue. These segments are then passed up to the HTTP application as quickly as HTTP can accept them.

Segments with port 25 specified are placed in a separate queue that is directed toward email services. In this manner, Transport Layer protocols enable servers at the ISP to host many different applications and services simultaneously.

In any Internet transaction, there is a source host and a destination host, normally a client and a server. The TCP processes on the sending and receiving hosts are slightly different. Clients are active and request connections, while servers are passive, and listen for and accept connections.

Server processes are usually statically assigned well-known port numbers from 0 to 1023. Well-known port numbers enable a client application to assign the correct destination port when generating a request for services.

Clients also require port numbers to identify the requesting client application. Source ports are dynamically assigned from the port range 1024 to 65535. This port assignment acts like a return address for the requesting application. The Transport Layer protocols keep track of the source port and the application that initiated the request, so that when a response is returned, it can be forwarded to the correct application.

The combination of the Transport Layer port number and the Network Layer IP address of the host uniquely identifies a particular application process running on an individual host device. This combination is called a socket. A socket pair, consisting of the source and destination IP addresses and port numbers, is also unique and identifies the specific conversation between the two hosts.

A client socket might look like this, with 7151 representing the source port number:

The socket on a web server might be:

Together, these two sockets combine to form a socket pair:,

With the creation of sockets, communication endpoints are known so that data can move from an application on one host to an application on another. Sockets enable multiple processes running on a client to distinguish themselves from each other, and multiple connections to a server process to be distinguished from each other.

TCP/IP Host Name

Communication between source and destination hosts over the Internet requires a valid IP address for each host. However, numeric IP addresses, especially the hundreds of thousands of addresses assigned to servers available over the Internet, are difficult for humans to remember. Human-readable domain names, like, are easier for people to use. Network naming systems are designed to translate human-readable names into machine-readable IP addresses that can be used to communicate over the network.

Humans use network naming systems every day when surfing the web or sending email messages, and may not even realize it. Naming systems work as a hidden but integral part of network communication. For example, to browse to the Cisco Systems website, open a browser and enter in the address field. The is a network name that is associated with a specific IP address. Typing the server IP address into the browser brings up the same web page.

Network naming systems are a human convenience to help users reach the resource they need without having to remember the complex IP address.

In the early days of the Internet, host names and IP addresses were managed through the use of a single HOSTS file located on a centrally administered server.

The central HOSTS file contained the mapping of the host name and IP address for every device connected to the early Internet. Each site could download the HOSTS file and use it to resolve host names on the network. When a host name was entered, the sending host would check the downloaded HOSTS file to obtain the IP address of the destination device.

At first, the HOSTS file was acceptable for the limited number of computer systems participating in the Internet. As the network grew, so did the number of hosts needing name-to-IP translations. It became impossible to keep the HOSTS file up to date. As a result, a new method to resolve host names to IP addresses was developed. DNS was created for domain name to address resolution. DNS uses a distributed set of servers to resolve the names associated with the numbered addresses. The single, centrally administered HOSTS file is no longer needed.

However, virtually all computer systems still maintain a local HOSTS file. A local HOSTS file is created when TCP/IP is loaded on a host device. As part of the name resolution process on a computer system, the HOSTS file is scanned even before the more robust DNS service is queried. A local HOSTS file can be used for troubleshooting or to override records found in a DNS server.

DNS Hierarchy

DNS solves the shortcomings of the HOSTS file. The structure of DNS is hierarchical, with a distributed database of host name to IP mappings spread across many DNS servers all over the world. This is unlike a HOSTS file, which requires all mappings to be maintained on one server.

DNS uses domain names to form the hierarchy. The naming structure is broken down into small, manageable zones. Each DNS server maintains a specific database file and is only responsible for managing name-to-IP mappings for that small portion of the entire DNS structure. When a DNS server receives a request for a name translation that is not within its DNS zone, the DNS server forwards the request to another DNS server within the proper zone for translation.

DNS is scalable because host name resolution is spread across multiple servers.

DNS is made up of three components.

Resource Records and Domain Namespace
A resource record is a data record in the database file of a DNS zone. It is used to identify a type of host, a host IP address, or a parameter of the DNS database.

The domain namespace refers to the hierarchical naming structure for organizing resource records. The domain namespace is made up of various domains, or groups, and the resource records within each group.

Domain Name System Servers
Domain name system servers maintain the databases that store resource records and information about the domain namespace structure. DNS servers attempt to resolve client queries using the domain namespace and resource records it maintains in its zone database files. If the name server does not have the requested information in its DNS zone database, it uses additional predefined name servers to help resolve the name-to-IP query.

Resolvers are applications or operating system functions that run on DNS clients and DNS servers. When a domain name is used, the resolver queries the DNS server to translate that name to an IP address. A resolver is loaded on a DNS client, and is used to create the DNS name query that is sent to a DNS server. Resolvers are also loaded on DNS servers. If the DNS server does not have the name-to-IP mapping requested, it uses the resolver to forward the request to another DNS server.

DNS uses a hierarchical system to provide name resolution. The hierarchy looks like an inverted tree, with the root at the top and branches below.

At the top of the hierarchy, the root servers maintain records about how to reach the top-level domain servers, which in turn have records that point to the second-level domain servers.

The different top-level domains represent either the type of organization or the country of origin. Examples of top-level domains are:

.au – Australia

.co – Colombia

.com – a business or industry

.jp – Japan

.org – a nonprofit organization

Under top-level domains are second-level domain names, and below them are other lower level domains.

The root DNS server may not know exactly where the host is located, but it does have a record for the .com top-level domain. Likewise, the servers within the .com domain may not have a record for either, but they do have a record for the domain. The DNS servers within the domain do have the record for and can resolve the address.

DNS relies on this hierarchy of decentralized servers to store and maintain these resource records. The resource records contain domain names that the server can resolve, and alternate servers that can also process requests.

The name is referred to as a fully qualified domain name (FQDN) or DNS name, because it defines the exact location of the computer within the hierarchical DNS namespace.

DNS Name Resolution

When a host needs to resolve a DNS name, it uses the resolver to contact a DNS server within its domain. The resolver knows the IP address of the DNS server to contact because it is preconfigured as part of the host IP configuration.

When the DNS server receives the request from the client resolver, it first checks the local DNS records it has cached in its memory. If it is unable to resolve the IP address locally, the server uses its resolver to forward the request to another preconfigured DNS server. This process continues until the IP address is resolved. The name resolution information is sent back to the original DNS server, which uses the information to respond to the initial query.

During the process of resolving a DNS name, each DNS server caches, or stores, the information it receives as replies to the queries. The cached information enables the DNS server to reply more quickly to subsequent resolver requests, because the server first checks the cache records before querying other DNS servers.

DNS servers only cache information for a limited amount of time. DNS servers should not cache information for too long because host name records do periodically change. If a DNS server had old information cached, it may give out the wrong IP address for a computer.

In the early implementations of DNS, resource records for hosts were all added and updated manually. However, as networks grew and the number of host records needing to be managed increased, it became very inefficient to maintain the resource records manually. Furthermore, when DHCP is used, the resource records within the DNS zone have to be updated even more frequently. To make updating the DNS zone information easier, the DNS protocol was changed to allow computer systems to update their own record in the DNS zone through dynamic updates.

Dynamic updates enable DNS client computers to register and dynamically update their resource records with a DNS server whenever changes occur. To use dynamic update, the DNS server and the DNS clients, or DHCP server, must support the dynamic update feature. Dynamic updates on the DNS server are not enabled by default, and must be explicitly enabled. Most current operating systems support the use of dynamic updates.

DNS servers maintain the zone database for a given portion of the overall DNS hierarchy. Resource records are stored within that DNS zone.

DNS zones can be either a forward lookup or reverse lookup zone. They can also be either a primary or a secondary forward or reverse lookup zone. Each zone type has a specific role within the overall DNS infrastructure.

Forward Lookup Zones
A forward lookup zone is a standard DNS zone that resolves fully qualified domain names to IP addresses. This is the zone type that is most commonly found when surfing the Internet. When typing a website address, such as, a recursive query is sent to the local DNS server to resolve that name to an IP address to connect to the remote web server.

Reverse Lookup Zones
A reverse lookup zone is a special zone type that resolves an IP address to a fully qualified domain name. Some applications use reverse lookups to identify computer systems that are actively communicating with them. There is an entire reverse lookup DNS hierarchy on the Internet that enables any publicly registered IP address to be resolved. Many private networks choose to implement their own local reverse lookup zones to help identify computer systems within their network. Reverse lookups on IP addresses can be found using the ping -a [ip_address] command.

Primary Zones
A primary DNS zone is a zone that can be modified. When a new resource record needs to be added or an existing record needs to be updated or deleted, the change is made on a primary DNS zone. When you have a primary zone on a DNS server, that server is said to be authoritative for that DNS zone, since it will have the answer for DNS queries for records within that zone. There can only be one primary DNS zone for any given DNS domain; however, you can have a primary forward and primary reverse lookup zone.

Secondary Zones
A secondary zone is a read-only backup zone maintained on a separate DNS server than the primary zone. The secondary zone is a copy of the primary zone and receives updates to the zone information from the primary server. Since the secondary zone is a read-only copy of the zone, all updates to the records need to be done on the corresponding primary zone. You can also have secondary zones for both forward and reverse lookup zones. Depending on the availability requirements for a DNS zone, you may have many secondary DNS zones spread across many DNS servers.

Implimenting DNS Solutions

There is more than one way to implement DNS solutions.

ISP DNS Servers
ISPs typically maintain caching-only DNS servers. These servers are configured to forward all name resolution requests to the root servers on the Internet. Results are cached and used to reply to any future requests. Because ISPs typically have many customers, the number of cached DNS lookups is high. The large cache reduces network bandwidth by reducing the frequency that DNS queries that are forwarded to the root servers. Caching-only servers do not maintain any authoritative zone information, meaning that they do not store any name-to-IP mappings directly within their database.

Local DNS Servers
A business may run its own DNS server. The client computers on that network are configured to point to the local DNS server rather than the ISP DNS server. The local DNS server may maintain some authoritative entries for that zone, so it has name-to-IP mappings of any host within the zone. If the DNS server receives a request that it cannot resolve, it is forwarded. The cache required on a local server is relatively small compared to the ISP DNS server because of the smaller number of requests.

It is possible to configure local DNS servers to forward requests directly to the root DNS server. However, some administrators configure local DNS servers to forward all DNS requests to an upstream DNS server, such as the DNS server of the ISP. In this way, the local DNS server benefits from the large number of cached DNS entries of the ISP, rather than having to go through the entire lookup process starting from the root server.

Losing access to DNS servers affects the visibility of public resources. If users type in a domain name that cannot be resolved, they cannot access the resource. For this reason, when an organization registers a domain name on the Internet, a minimum of two DNS servers must be provided with the registration. These servers are the ones that hold the DNS zone database. Redundant DNS servers ensure that if one fails, the other one is available for name resolution. This practice provides fault tolerance. If hardware resources permit, having more than two DNS servers within a zone provides additional protection and organization.

It is also a good idea to make sure that multiple DNS servers that host the zone information are located on different physical networks. For example, the primary DNS zone information can be stored on a DNS server on the local business premises. Usually the ISP hosts an additional secondary DNS server to ensure fault tolerance.

DNS is a critical network service. Therefore, DNS servers must be protected using firewalls and other security measures. If DNS fails, other web services are not accessible.


In addition to providing private and business customers with connectivity and DNS services, ISPs provide many business-oriented services to customers. These services are enabled by software installed on servers. Among the different services provided by ISPs are:
– email hosting
– website hosting
– e-commerce sites
– file storage and transfer
– message boards and blogs
– streaming video and audio services

TCP/IP Application Layer protocols enable many of these ISP services and applications. The most common TCP/IP Application Layer protocols are HTTP, FTP, SMTP, POP3, and IMAP4.

Some customers have greater concern about security, so these Application Layer protocols also include secure versions such as FTPS and HTTPS.


HTTP, one of the protocols in the TCP/IP suite, was originally developed to enable the retrieval of HTML-formatted web pages. It is now used for distributed, collaborative information sharing. HTTP has evolved through multiple versions. Most ISPs use HTTP version 1.1 to provide web-hosting services. Unlike earlier versions, version 1.1 enables a single web server to host multiple websites. It also permits persistent connections, so that multiple request and response messages can use the same connection, reducing the time it takes to initiate new TCP sessions.

HTTP specifies a request/response protocol. When a client, typically a web browser, sends a request message to a server, HTTP defines the message types that the client uses to request the web page. It also defines the message types that the server uses to respond.

Although it is remarkably flexible, HTTP is not a secure protocol. The request messages send information to the server in plain text that can be intercepted and read. Similarly, the server responses, typically HTML pages, are also sent unencrypted.

For secure communication across the Internet, Secure HTTP (HTTPS) is used for accessing or posting web server information. HTTPS can use authentication and encryption to secure data as it travels between the client and server. HTTPS specifies additional rules for passing data between the Application Layer and the Transport Layer.

When contacting an HTTP server to download a web page, a uniform resource locator (URL) is used to locate the server and a specific resource. The URL identifies:
– Protocol being used
– Domain name of the server being accessed
– Location of the resource on the server, such as

Many web server applications allow short URLs. Short URLs are popular because they are easier to write down, remember, or share. With a short URL, a default resource page is assumed when a specific URL is typed. When a user types in a shortened URL, like, the default page that is sent to the user is actually the web page.

HTTP supports proxy services. A proxy server allows clients to make indirect network connections to other network services. A proxy is a device in the communications stream that acts as a server to the client and as a client to a server.

The client connects to the proxy server and requests from the proxy a resource on a different server. The proxy connects to the specified server and retrieves the requested resource. It then forwards the resource back to the client.

The proxy server can cache the resulting page or resource for a configurable amount of time. Caching enables future clients to access the web page quickly, without having to access the actual server where the page is stored. Proxies are used for three reasons:
– Speed – Caching allows resources requested by one user to be available to subsequent users, without having to access the actual server where the page is stored.
– Security – Proxy servers can be used to intercept computer viruses and other malicious content and prevent them from being forwarded onto clients.
– Filtering – Proxy servers can view incoming HTTP messages and filter unsuitable and offensive web content.

HTTP sends clear text messages back and forth between a client and a server. These text messages can be easily intercepted and read by unauthorized users. To safeguard data, especially confidential information, some ISPs provide secure web services by using HTTPS. HTTPS is HTTP over secure socket layer (SSL). HTTPS uses the same client request-server response process as HTTP, but the data stream is encrypted with SSL before being transported across the network.

When the HTTP data stream arrives at the server, the TCP layer passes it up to SSL in the Application Layer of the server, where it is decrypted.

The maximum number of simultaneous connections that a server can support for HTTPS is less than that for HTTP. HTTPS creates additional load and processing time on the server due to the encryption and decryption of traffic. To keep server performance up, HTTPS should only be used when necessary, such as when exchanging confidential information.


FTP is a connection-oriented protocol that uses TCP to communicate between a client FTP process and an FTP process on a server. FTP implementations include the functions of a protocol interpreter (PI) and a data transfer process (DTP). PI and DTP define two separate processes that work together to transfer files. As a result, FTP requires two connections to exist between the client and server, one to send control information and commands, and a second one for the actual file data transfer.

Protocol Interpreter (PI)
The PI function is the main control connection between the FTP client and the FTP server. It establishes the TCP connection and passes control information to the server. Control information includes commands to navigate through a file hierarchy and renaming or moving files. The control connection, or control stream, stays open until closed by the user. When a user wants to connect to an FTP server there are five basic steps:
– Step 1. The user PI sends a connection request to the server PI on well-known port 21.
– Step 2. The server PI replies and the connection is established.
– Step 3. With the TCP control connection open, the server PI process begins the login sequence.
– Step 4. The user enters credentials through the user interface and completes authentication.
– Step 5. The data transfer process begins.

Data Transfer Process
DTP is a separate data transfer function. This function is enabled only when the user wants to actually transfer files to or from the FTP server. Unlike the PI connection, which remains open, the DTP connection closes automatically when the file transfer is complete.

The two types of data transfer connections supported by FTP are active data connections and passive data connections.

Active Data Connections
In an active data connection, a client initiates a request to the server and opens a port for the expected data. The server then connects to the client on that port and the file transfer begins.

Passive Data Connections
In a passive data connection, the FTP server opens a random source port (greater than 1023). The server forwards its IP address and the random port number to the FTP client over the control stream. The server then waits for a connection from the FTP client to begin the data file transfer.

ISPs typically support passive data connections to their FTP servers. Firewalls often do not permit active FTP connections to hosts located on the inside network.


One of the primary services offered by an ISP is email hosting. Email is a store-and-forward method of sending, storing, and retrieving electronic messages across a network. Email messages are stored in databases on mail servers. ISPs often maintain mail servers that support many different customer accounts.

Email clients communicate with mail servers to send and receive email. Mail servers communicate with other mail servers to transport messages from one domain to another. An email client does not communicate directly with another email client when sending email. Instead, both clients rely on the mail server to transport messages. This is true even when both users are in the same domain.

Email clients send messages to the email server configured in the application settings. When the server receives the message, it checks to see if the recipient domain is located on its local database. If it is not, it sends a DNS request to determine the mail server for the destination domain. When the IP address of the destination mail server is known, the email is sent to the appropriate server.

Email supports three separate protocols for operation: SMTP, POP3, and IMAP4. The Application Layer process that sends mail, either from a client to a server or between servers, implements SMTP. A client retrieves email using one of two Application Layer protocols: POP3 or IMAP4.

SMTP transfers mail reliably and efficiently. For SMTP applications to work properly, the mail message must be formatted properly and SMTP processes must be running on both the client and server.

SMTP message formats require a message header and a message body. While the message body can contain any amount of text, the message header must have a properly formatted recipient email address and a sender address. Any other header information is optional.

When a client sends email, the client SMTP process connects with a server SMTP process on well-known port 25. After the connection is made, the client attempts to send mail to the server across the connection. When the server receives the message, it either places the message in a local account or forwards the message using the same SMTP connection process to another mail server.

The destination email server may not be online or may be busy when email messages are sent. Therefore, SMTP spools messages to be sent at a later time. Periodically, the server checks the queue for messages and attempts to send them again. If the message is still not delivered after a predetermined expiration time, it is returned to the sender as undeliverable.

One of the required fields in an email message header is the recipient email address. The structure of an email address includes the email account name or an alias, in addition to the domain name of the mail server. An example of an email address:

The @ symbol separates the account and the domain name of the server. When a DNS server receives a query for a name with an @ symbol, that indicates to the DNS server that it is looking up an IP address for a mail server.

When a message is sent to, the domain name is sent to the DNS server to obtain the IP address of the domain mail server. Mail servers are identified in DNS by an MX record indicator. MX is a type of resource record stored on the DNS server. When the destination mail server receives the message, it stores the message in the appropriate mailbox. The mailbox location is determined based on the account specified in the first part of the email address, in this case, the recipient account. The message remains in the mailbox until the recipient connects to the server to retrieve the email.

If the mail server receives an email message that references an account that does not exist, the email is returned to the sender as undeliverable.

Post Office Protocol – Version 3 (POP3) enables a workstation to retrieve mail from a mail server. With POP3, mail is downloaded from the server to the client and then deleted on the server.

The server starts the POP3 service by passively listening on TCP port 110 for client connection requests. When a client wants to make use of the service, it sends a request to establish a TCP connection with the server. When the connection is established, the POP3 server sends a greeting. The client and POP3 server then exchange commands and responses until the connection is closed or aborted.

Because email messages are downloaded to the client and removed from the server, there is not a centralized location where email messages are kept. Because POP3 does not store messages, it is undesirable for a small business that needs a centralized backup solution.

POP3 is desirable for an ISP, because it alleviates their responsibility for managing large amounts of storage for their email servers.

Internet Message Access Protocol (IMAP4) is another protocol that describes a method to retrieve email messages. However, unlike POP3, when the user connects to an IMAP-capable server, copies of the messages are downloaded to the client application. The original messages are kept on the server until manually deleted. Users view copies of the messages in their email client software.

Users can create a file hierarchy on the server to organize and store mail. That file structure is duplicated on the email client as well. When a user decides to delete a message, the server synchronizes that action and deletes the message from the server.

For small- to medium-sized businesses, there are many advantages to using IMAP. IMAP can provide long-term storage of email messages on mail servers and allows for centralized backup. It also enables employees to access email messages from multiple locations, using different devices or client software. The mailbox folder structure that a user expects to see is available for viewing regardless of how the user accesses the mailbox.

For an ISP, IMAP may not be the protocol of choice. It can be expensive to purchase and maintain the disk space to support the large number of stored emails. Additionally, if customers expect their mailboxes to be backed up routinely, that can further increase the costs to the ISP.

Chapter 6 – Routing

Filed under: Uncategorized — krystalchisholm @ 9:17 am

Routing Basics

As the internal network of an organization grows, it may be necessary to break up the network into multiple smaller networks for security or organizational purposes. This division is often accomplished by subnetting the network. Subnetting requires a router to pass traffic from one subnet to another.

To direct messages across networks so that they arrive at the correct destination, a router uses a table containing all the locally connected networks and the interfaces that are connected to each network. Each interface belongs to a different IP network.

A router determines which route, or path, to use by looking up the information stored in its routing table. The routing table also contains information about routes that the router can use to reach remote networks which are not locally attached.

Routes can be statically assigned to a router by an administrator, or routes can be dynamically given to the router by another router via a routing protocol.

A router uses a routing table to determine where to send packets. The routing table contains a set of routes. Each route describes which gateway or interface the router uses to reach a specified network.
A route has four main components:
– Destination value
– Subnet mask
– Gateway or interface address
– Route cost or metric

When a router receives a packet, the router examines the destination IP address in that packet to determine where to forward the packet. The router then looks for a matching destination value in the routing table.

Each destination value within the route table refers to a destination network address. The destination IP address within a packet, however, consists of both a network address and a host address. For the router to determine if its table contains a route to the destination network, it must determine there is a match between the IP network address and one of the destination values in the routing table. This means the router must determine which bits of the IP address represent the network and which bits represent the host.

The router looks up the subnet mask assigned to each potential route in the table. The router applies each subnet mask to the destination IP address in the packet. The resulting network address is then compared to the network address of the route in the table. If a match is found, the packet is forwarded out the correct interface or to the appropriate gateway. If the network address matches more than one route in the routing table, the router uses the route that has the most specific, or longest, network address match.

Sometimes there is more than one route to the destination network. In this case, routing protocol rules determine which route the router uses.
If none of the route entries match, the router directs the message to the gateway specified by its default route, if a default route is configured. Otherwise, the packet is simply dropped.

On a Cisco router, the Cisco IOS command show ip route displays the routes in the routing table. Several types of routes can appear in the routing table.

Directly Connected Routes
When the router powers up, the configured interfaces are enabled. As the interfaces become operational, the router stores the directly attached, local-network addresses as connected routes in the routing table. On Cisco routers, these routes are identified in the routing table with the prefix C. The routes are automatically updated whenever the interface is reconfigured or shut down.

Static Routes
A network administrator can manually configure a static route to a specific network. A static route does not change until the administrator manually reconfigures it. These routes are identified in the routing table with the prefix S.

Dynamically Updated Routes (Dynamic Routes)
Dynamic routes are automatically created and maintained by routing protocols. Routing protocols exchange routing information with other routers in the network. Dynamically updated routes are identified in the routing table with the prefix that corresponds to the type of routing protocol that created the route. For example, R is used for the Routing Information Protocol (RIP).

Default Route
The default route is a type of static route that specifies the gateway to use when the routing table does not contain a path for the destination network. It is common for default routes to point to the next router in the path to the ISP. If a subnet has only one router, that router is automatically the default gateway, because all network traffic to and from that local network has no option but to travel through that router.

Routing tables do not contain end-to-end information about the entire path from a source network to a destination network. They only contain information about the next hop along that path. The next hop is typically a directly-connected network within the routing table.

In the case of a static route, the next hop could be any IP address, as long as it is reachable by that router. Eventually the message gets passed to a router that is directly connected to the destination host and the message is delivered. Routing information between all the intermediate routers on a path is in the form of network addresses not specific hosts. It is only in the final router that the destination address in the routing table points specifically to a host computer rather than a network.

Configuring Static Routes

Static routes are manually configured by a network administrator. Configuring a static route on a Cisco router requires these steps:
– Step 1. Connect to the router using a console cable.
– Step 2. Open a HyperTerminal window to connect with the first router that you want to configure.
– Step 3. Enter privileged mode by typing enable at the R1> prompt. Note how the > symbol changes to a # to indicate that privilege mode is being used.
– Step 4. Enter global configuration mode.
R1#config terminal
– Step 5. Use the ip route Cisco IOS command to configure the static route, with the following format.
ip route [destination_network] [subnet_mask] [gateway_address]
ip route [destination_network] [subnet_mask] [exit_interface]
For example, to enable router 1 (R1) to reach a host on network, the administrator configures a static route on R1 with the following Cisco IOS command in global configuration mode:
R1(config)#ip route
R1(config)#ip route S0/0/0
To enable two-way communication with a host on network, the administrator also configures a static route on router 2 (R2).

Because static routes are configured manually, network administrators must add and delete static routes to reflect any changes in network topology. On small networks, static routes require very little maintenance because there are not many possible changes. In a large network, manually maintaining routing tables could require significant administrative time. For this reason, larger networks generally use dynamic routing rather than static routes.

Routing Protocols

Routes can change very quickly. Problems with cables and hardware failures can make destinations unreachable through the designated interface. Routers need to be able to quickly update routes in a way that does not depend on the administrator to make the changes manually.

Routers use routing protocols to dynamically manage information received from their own interfaces and from other routers. Routing protocols can also be configured to manage manually entered routes.

Dynamic routing makes it possible to avoid the time-consuming process of configuring static routes. Dynamic routing enables routers to react to changes in the network and to adjust their routing tables accordingly, without the intervention of the network administrator.

A dynamic routing protocol learns all the available routes, places the best routes into the routing table, and removes routes when they are no longer valid. The method that a routing protocol uses to determine the best route is called a routing algorithm. There are two main classes of routing algorithms: distance vector and link state. Each type uses a different method for determining the best route to a destination network.

Whenever the topology of a network changes because of reconfiguration or failure, the routing tables in all the routers must also change to reflect an accurate view of the new topology. When all the routers in a network have updated their tables to reflect the new route, the routers are said to have converged.

The specific routing algorithm that is being used is a very important factor in dynamic routing. For two routers to exchange routes, they must be using the same routing protocol and therefore the same routing algorithm.

The distance vector routing algorithm periodically passes copies of the routing table from router to router. These regular updates between routers communicate topology changes.

The distance vector algorithm evaluates the route information it receives from other routers using two basic criteria:
– Distance – How far away is the network from this router?
– Vector – In which direction should the packet be sent to reach this network?

The distance component of a route is expressed in terms of a route cost, or metric, that can be based on the following items:
– Number of hops
– Administrative cost
– Bandwidth
– Transmission speed
– Likelihood of delays
– Reliability

The vector, or direction, component of a route is the address of the next hop along the path to the network named in the route.

An analogy for distance vectors are the highway signs found at intersections. A sign points toward a destination and indicates the distance that must be traveled to reach that destination. Further down the highway, another sign points toward the same destination, but now the distance remaining to that destination is shorter. As long as the distance is shorter, the traffic is on the best path.

Each router that uses distance vector routing communicates its routing information to its neighbors. Neighbor routers share a directly connected network. The interface that leads to each directly connected network has a distance of 0.

Each router receives a routing table from its neighbor routers. For example, R2 receives information from R1. R2 adds to the metric, in this case the hop count, to show that there is now one more hop to get to the destination network. Then R2 sends this new routing table to its neighbors, including R3. This step-by-step process occurs in all directions between neighbor routers.

Eventually, each router learns about other more-remote networks based on the information that it receives from its neighbors. Each of the network entries in the routing table has an accumulated distance vector to show how far away that network is in a given direction.

As the distance vector discovery process continues, routers discover the best path to destination networks based on the information they receive from each neighbor. The best path is the path with the shortest distance or smallest metric.

Routing table updates also occur when the topology changes, for example, when a new network is added or when a router fails, causing a network to become unreachable. As with the network discovery process, topology change updates proceed step-by-step by sending copies of routing tables from router to router.

Common Interior Routing Protocols

Routing Information Protocol (RIP) is a distance vector routing protocol that is used in thousands of networks throughout the world. It was initially specified in RFC 1058.

Characteristics of RIP include:
– Is a distance vector routing protocol
– Uses hop count as the metric for path selection
– Defines a hop count greater than 15 as an unreachable route
– Sends routing table contents every 30 seconds

When a router receives a routing update with a change, it updates its routing table to reflect that change. If the router learns a new route from another router, it increases the hop count value by one before adding that route to its own routing table. The router uses the local network address of the directly connected router that sent the update as the next hop address.

After updating its routing table, the router immediately begins transmitting routing updates to inform other network routers of the change. These updates, called triggered updates, are sent independently of the regularly scheduled updates that RIP routers forward.

Routing Information Protocol (RIP)

RIP is simple and easy to implement. These advantages make RIP a widely used and popular routing protocol.

RIP has several disadvantages:
– Allows a maximum of 15 hops, so it can only be used for networks that connect no more than 16 routers in a series.
– Periodically sends complete copies of the entire routing table to directly connected neighbors. In a large network, this can cause a significant amount of network traffic each time there is an update.
– Converges slowly on larger networks when the network changes.

There are currently two versions of RIP available: RIPv1 and RIPv2. RIPv2 has many advantages over RIPv1 and is usually used unless the equipment cannot support RIPv2. The most significant difference between RIP versions 1 and 2 is that RIPv2 can support classless routing, because it includes the subnet mask information in routing updates. RIPv1 does not send subnet mask information in the updates; therefore, it must rely on the classful default subnet masks.

Enhanced Interior Gateway Routing Protocol (EIGRP)

EIGRP is a Cisco-proprietary, enhanced distance vector routing protocol. EIGRP was developed to address some of the limitations of other distance vector routing protocols, such as RIP. These limitations include the use of the hop count metric and the maximum network size of 15 hops.

EIGRP uses a number of metrics, including a configured bandwidth value and the delay encountered when a packet travels a particular route.

The characteristics of EIGRP are:
– Uses a variety of metrics to calculate the cost of a route
– Combines the next hop and metric features of distance vector protocols with additional database and update features
– Has a maximum hop count of 224 hops

Unlike RIP, EIGRP does not rely only on the routing table in the router to hold all the information it needs to operate. EIGRP creates two additional database tables: the neighbor table and the topology table.

The neighbor table stores data about the neighboring routers that are on directly connected local networks. This neighbor table includes information such as the interface IP addresses, interface type, and bandwidth.

EIGRP builds the topology table from each of the advertisements of its neighbors. The topology table contains all the routes advertised by the neighbor routers. EIGRP depends on a routing algorithm called Diffused Update Algorithm (DUAL) to calculate the shortest path to a destination within a network and to install this route into the routing table. The topology table enables a router running EIGRP to find the best alternate path quickly when a network change occurs. If no alternate route exists in the topology table, EIGRP queries its neighbors to find a new path to the destination.

Unlike RIP, which is limited to small simple networks of less than 15 hops, EIGRP is ideal for larger, more complex networks up to 224 hops in size that require fast convergence.

Link-state Protocol

Routers that use the distance vector routing algorithm have little information about distant networks and none about distant routers. The link-state routing algorithm maintains a full database of distant routers and how they interconnect.

Link-state routing uses the following features:
– Routing table – List of the known paths and interfaces.
– Link-state advertisement (LSA) – Small packet of routing information that is sent between routers. LSAs describe the state of the interfaces (links) of a router and other information, such as the IP address of each link.
– Topological database – Collection of information gathered from all the LSAs received by the router.
– Shortest Path First (SPF) algorithm – Calculation performed on the database that results in the SPF tree. The SPF tree is a map of the network as seen from the point of view of the router. The information in this tree is used to build the routing table.

When LSAs are received from other routers, the SPF algorithm analyzes the information in the database to construct the SPF tree. Based on the SPF tree, the SPF algorithm then calculates the shortest paths to other networks. Each time a new LSA packet causes a change to the link-state database, SPF recalculates the best paths and updates the routing table.


Open Shortest Path First (OSPF) is a non-proprietary, link-state routing protocol described in RFC 2328. The characteristics of OSPF are:
– Uses the SPF algorithm to calculate the lowest cost to a destination
– Sends routing updates only when the topology changes; does not send periodic updates of the entire routing table
– Provides fast convergence
– Supports Variable Length Subnet Mask (VLSM) and discontiguous subnets
– Provides route authentication

In OSPF networks, routers send link-state advertisements to each other when a change occurs, for example, when a new neighbor is added, or when a link fails or is restored.

If the network topology changes, the routers affected by the change send update LSAs to the rest of the network. All routers update their topology databases accordingly, regenerate their SPF trees to find new shortest paths to each network, and update their routing tables with the changed routes.

OSPF requires more router resources, such as RAM and CPU processing power, and is an advanced networking protocol that requires an experienced support staff.

Each routing protocol uses different metrics. The metric used by one routing protocol is not comparable to the metric used by another routing protocol. Two routing protocols might choose different paths to the same destination because they use different metrics. For example, RIP chooses the path with the fewest number of hops, whereas EIGRP chooses the path based on the highest bandwidth and least delay.

Metrics used in IP routing protocols include:
– Hop count – Number of routers a packet must traverse.
– Bandwidth – Bandwidth of a specific link.
– Load – Traffic utilization of a specific link.
– Delay – Time a packet takes to traverse a path.
– Reliability — Probability of a link failure, based on the interface error count or previous link failures.
– Cost – Determined by either the Cisco IOS application or the network administrator to indicate preference for a route. Cost can represent a metric, a combination of metrics, or a policy.

It is possible to have more than one routing protocol enabled on a single router. Additionally, a network administrator may choose to configure static routes to a specific destination. If a router has two different paths to a destination based on two different routing protocols and their metrics, how does the router know which path to use?

The router uses what is known as the administrative distance (AD). The AD represents the “trustworthiness” of the route. The lower the AD, the more the trustworthy the route. For example, a static route has an AD of 1, whereas a RIP-discovered route has an AD of 120. Given two separate routes to the same destination, the router chooses the route with the lowest AD. When a router has the choice of a static route and a RIP route, the static route takes precedence. Additionally, a directly connected route with an AD of 0 takes precedence over a static route with an AD of 1.

Sometimes it is necessary to use multiple routing protocols, for example, when merging two pre-existing networks. However, when initially designing a network, it is recommended that only one routing protocol be enabled for the entire network. Having one protocol makes it easier to support and troubleshoot the network. Deciding which type of routing protocol to select can be difficult even for expert network designers.

Small networks with only one gateway to the Internet can probably use static routes. Such a topology rarely needs dynamic routing.

As an organization grows and adds routers to its network topology, RIPv2 can be used. It is easy to configure and works well in small networks. When a network begins to exceed 15 routers, RIP is no longer a good choice.

For larger networks, EIGRP and OSPF are commonly used, but there is no simple principle that makes it obvious to choose one over the other. Each network has to be considered independently. The three main criteria to consider are:
– Ease of management – What information does the protocol keep about itself? Which show commands are available?
– Ease of configuration – How many commands does the average configuration require? Is it possible to configure several routers in the network with the same configuration?
– Efficiency – How much bandwidth does the routing protocol use while it is in a steady state, and how much could it use when converging in response to a major network event?

Configuring & Veryfying RIP

RIP is a popular distance vector protocol supported by most routers. It is an appropriate choice for small networks containing multiple routers. Before configuring RIP on a router, think about the networks a router serves, and the interfaces on the router that connect to these networks.

The figure shows three routers. Each router serves a separate private local network, so there are three LANs. The routers are also connected by separate networks, so there are a total of six networks shown.

With this topology, R1 does not automatically know how to reach the network, or the network. R1 is only able to reach those networks after RIP routing is properly configured. Once RIP routing is configured, R2 and R3 will forward routing updates to R1 containing information on the availability of the and networks.

Before configuring RIP, assign an IP address and enable all the physical interfaces that will participate in routing.

For the most basic RIPv2 configuration, there are three commands to remember:
Router(config)#router rip
Router (config-router)#version 2
Router(config-router)#network [network_number]

Enter the router rip command in global configuration mode to enable RIP on the router. Enter the network command from router configuration mode to tell the router which networks are part of the RIP routing process. The routing process associates specific interfaces with the network numbers specified, and begins to send and receive RIP updates on these interfaces.

After a configuration is done, it is a good idea to compare the running configuration with an accurate topology diagram to verify the network numbers and interface IP addresses. This is good practice because it is easy to make a simple data entry error.

There are several ways to verify that RIP is functioning properly in the network. One way to verify that routing is working properly is to ping devices on remote networks. If the ping is successful, it is likely that routing is working.

Another method is to run the IP routing verification commands show ip protocols and show ip route at the CLI prompt.

The show ip protocols command verifies that RIP routing is configured, that the correct interfaces are sending and receiving RIP updates, and that the router is advertising the correct networks.

The show ip route command shows the routing table, which verifies that routes received by RIP neighbors are installed in the routing table.

The debug ip rip command can be used to observe the networks advertised in the routing updates as they are sent and received. Debug commands display router activity in real time. Because debug activity uses router processor resources, debugging should be used with care in a production network, because it can affect network operation.

Autonomous Systems

The Internet routing architecture has evolved over the years into a distributed system of interconnected networks. The Internet is now so vast and involves so many networks that it is impossible for a single organization to manage all the routing information needed to reach every destination around the world.

Instead, the Internet is divided up into collections of networks called Autonomous Systems (AS), which are independently controlled by different organizations and companies.

An AS is a set of networks controlled by a single administrative authority using the same internal routing policy throughout. Each AS is identified by a unique AS number (ASN). ASNs are controlled and registered on the Internet.

The most common example of an AS is the ISP. Most businesses connect to the Internet through an ISP, and so become part of the routing domain of that ISP. The AS is administered by the ISP and, therefore, not only includes its own network routes but also manages the routes to all the business and other customer networks that are connected to it.

The same ASN applies to all network devices within the AS routing domain.

ISP A is an AS whose routing domain includes a local business that directly connects to that ISP for Internet access. The business does not have a separate ASN. Instead, it uses the ASN of ISP A (ASN 100) in its routing information.

Also shown is a large global business with corporate offices located in Hong Kong and New York. Because they are located in different countries, each office connects to a different local ISP for Internet access. This means that the business is connected to two ISPs. Which AS does it belong to and which ASN does it use?

Because the company communicates through both ISP B and ISP C, this causes routing confusion in terms of connectivity. Traffic from the internet does not know which AS to use to reach the large global business. To solve the problem, the business registers as an AS in its own right and is assigned an ASN of 400.

Routing Across the Internet

Interior Gateway Protocols (IGPs) are used to exchange routing information within an AS or individual organization. The purpose of an interior routing protocol is to find the best path through the internal network. IGPs run on the routers inside an organization. Examples of IGPs are RIP, EIGRP, and OSPF.

By contrast, exterior gateway protocols (EGPs) are designed to exchange routing information between different autonomous systems. Because each AS is managed by a different administration and may use different interior protocols, networks must use a protocol that can communicate between diverse systems. The EGP serves as a translator for ensuring that external routing information gets successfully interpreted inside each AS network.

EGPs run on the exterior routers. These are the routers that are located at the border of an AS. Exterior routers are also called border gateways, or boundary routers.

Unlike interior routers, which exchange individual routes with each other using IGPs, exterior routers exchange information about how to reach various networks using exterior protocols. Exterior routing protocols seek to find the best path through the Internet as a sequence of autonomous systems.

The most common exterior routing protocol on the Internet today is Border Gateway Protocol (BGP). It is estimated that 95% of autonomous systems use BGP. The most current version of BGP is version 4 (BGP-4), for which the latest description is provided in RFC 4271.

Each AS is responsible for informing other autonomous systems about which networks they can reach through that AS. Autonomous systems exchange this reachability information with each other through exterior routing protocols that run on dedicated routers called border gateways.

Packets are routed across the Internet in several steps.
1. The source host sends a packet destined for a remote host located in another AS.
2. Because the destination IP address of the packet is not a local network, the interior routers keep passing the packet along their default routes, until eventually it arrives at an exterior router at the edge of the local AS.
3. The exterior router maintains a database for all the autonomous systems with which it connects. This reachability database tells the router that the path to the destination network passes through several autonomous systems, and that the next hop on the path is through a directly connected exterior router on a neighboring AS.
4. The exterior router directs the packet to its next hop on the path, which is the exterior router at the neighboring AS.
5. The packet arrives at the neighboring AS, where the exterior router checks its own reachability database and forwards the packet to the next AS on the path.
6. The process is repeated at each AS until the exterior router at the destination AS recognizes the destination IP address of the packet as an internal network in that AS.
7. The final exterior router then directs the packet to the next hop interior router listed in its routing table. From then on, the packet is treated just like any local packet and is directed through interior routing protocols through a series of internal next hops until it arrives at the destination host.

Exterior Routing Protocols

EGPs provide many useful features for ISPs. Exterior protocols allow traffic to be routed across the Internet to remote destinations. They also provide the method by which ISPs can set and enforce policies and local preferences so that the traffic flow through the ISP is efficient and that none of the internal routes are overloaded with transit traffic.

Business customers insist on reliability for their Internet service. ISPs must make sure that the Internet connection for those customers is always available. They do this by providing backup routes and routers in case the regular route fails. During normal conditions, the ISP advertises the regular route to other autonomous systems. If that regular route fails, the ISP sends an exterior protocol update message to advertise the backup route instead.

The flow of messages in the Internet is called traffic. Internet traffic can be categorized in one of two ways:
– Local traffic – Traffic carried within an AS that either originated in that same AS, or is intended to be delivered within that AS. This is like local traffic on a street.
– Transit traffic – Traffic that was generated outside that AS and can travel through the internal AS network to be delivered to destinations outside the AS. This is like through traffic on a street.

The flow of traffic between autonomous systems is carefully controlled. It is important to be able to limit or even prohibit certain types of messages from going to or from an AS for security reasons or to prevent overloading.

Many autonomous systems network administrators choose not to carry transit traffic. Transit traffic can cause routers to overload and fail if those routers do not have the capacity to handle large amounts of traffic.

Configuring & Verifying BGP

When an ISP puts a router at a customer location, they usually configure it with a default static route to the ISP. Sometimes, an ISP may want the router to be included in its AS and to participate in BGP. In these instances, it is necessary to configure the customer premise router with the commands necessary to enable BGP.

The first step in enabling BGP on a router is to configure the AS number. This step is done with the command:
router bgp [AS_number]

The next step is to identify the ISP router that is the BGP neighbor with which the customer premises equipment (CPE) router exchanges information. The command to identify the neighbor router is:
neighbor [IP_address] remote-as [AS_number]

When an ISP customer has its own registered IP address block, it may want the routes to some of its internal networks to be known on the Internet. To use BGP to advertise an internal route, the network address needs to be identified. The format of the command is:
network [network_address]

When the CPE is installed and the routing protocols are configured, the customer has both local and Internet connectivity. Now the customer is able to fully participate in other services that the ISP offers.

The IP addresses used for BGP are normally registered, routable addresses that identify unique organizations. In very large organizations, private addresses may be used in the BGP process. On the Internet, BGP should never be used to advertise a private network address.

November 17, 2010

Chapter 5 – Configuring Network Devices

Filed under: Uncategorized — krystalchisholm @ 10:34 am

Initial ISR Router Configurations

The Cisco Integrated Services Router (ISR) is one of the most popular networking devices to meet the growing communications needs of businesses. The ISR combines features such as routing and LAN switching functions, security, voice, and WAN connectivity into a single device. This makes the ISR ideal for small to medium-sized businesses and for ISP-managed customers.
The optional integrated switch module allows small businesses to connect LAN devices directly to the 1841 ISR. With the integrated switch module, if the number of LAN hosts exceeds the number of switch ports, additional switches or hubs can be connected in a daisy chain to extend the number of LAN ports available. If the switch module is not included, external switches are connected to the router interfaces of the ISR.
The ISR routing function allows a network to be broken into multiple local networks using subnetting and supports internal LAN devices connecting to the Internet or WAN.

The Cisco Internetwork Operating System (IOS) software provides features that enable a Cisco device to send and receive network traffic using a wired or wireless network. Cisco IOS software is offered to customers in modules called images. These images support various features for businesses of every size.
The entry-level Cisco IOS software image is called the IP Base image. The Cisco IOS IP Base software supports small to medium-sized businesses and supports routing between networks.

Other Cisco IOS software images add services to the IP Base image. For example, the Advanced Security image provides advanced security features, such as private networking and firewalls.
Many different types and versions of Cisco IOS images are available. Images are designed to operate on specific models of routers, switches, and ISRs.

It is important to know which image and version is loaded on a device before beginning the configuration process.

Physical ISR Setup

Each ISR is shipped with the cables and documentation needed to power up the device and begin the installation. When a new device is received, it is necessary to unpack the device and verify that all the hardware and equipment is included.

Items shipped with a new Cisco 1841 ISR include:
– RJ-45 to DB-9 console cable
– DB-9 to DB-25 modem adapter
– Power cord
– Product registration card, called the card
– Regulatory compliance and safety information for Cisco 1841 routers
– Router and Security Device Manager (SDM) Quick Start guide
– Cisco 1800 Series Integrated Services Router (Modular) Quick Start guide

To install a new Cisco 1841 ISR requires special tools and equipment, which most ISPs and technician labs usually have available. Any additional equipment required depends on the model of the device and any optional equipment ordered.

Typically, the tools required to install a new device include:
– PC with a terminal emulation program, such as HyperTerminal
– Cable ties and a No. 2 Phillips screwdriver
– Cables for WAN interfaces, LAN interfaces, and USB interfaces

It may also be necessary to have equipment and devices required for WAN and broadband communication services, such as a modem. Additionally, Ethernet switches may be required to connect LAN devices or expand LAN connectivity, depending on whether the integrated switch module is included and the number of LAN ports required.

Before beginning any equipment installation, be sure to read the Quick Start guide and other documentation that is included with the device. The documentation contains important safety and procedural information to prevent accidental damage to the equipment during installation.

Follow these steps to power up an 1841 ISR.
1. Securely mount and ground the device chassis, or case.
2. Seat the external compact flash card.
3. Connect the power cable.
4. Configure the terminal emulation software on the PC and connect the PC to the console port.
5. Turn on the router.
6. Observe the startup messages on the PC as the router boots up.

The router bootup process has three stages.

1. Perform Power-on self test (POST) and load the bootstrap program.
The POST is a process that occurs on almost every computer when it boots up. POST is used to test the router hardware. After POST, the bootstrap program is loaded.

2. Locate and load the Cisco IOS software.
The bootstrap program locates the Cisco IOS software and loads it into RAM. Cisco IOS files can be located in one of three places: flash memory, a TFTP server, or another location indicated in the startup configuration file. By default, the Cisco IOS software loads from flash memory. The configuration settings must be changed to load from one of the other locations.

3. Locate and execute the startup configuration file or enter setup mode.
After the Cisco IOS software is loaded, the bootstrap program searches for the startup configuration file in NVRAM. This file contains the previously saved configuration commands and parameters, including interface addresses, routing information, passwords, and other configuration parameters.

If a configuration file is not found, the router prompts the user to enter setup mode to begin the configuration process.
If a startup configuration file is found, it is copied into RAM and a prompt containing the host name is displayed. The prompt indicates that the router has successfully loaded the Cisco IOS software and configuration file.

To avoid the loss of data, it is important to have a clear understanding of the difference between the startup configuration file and the running configuration file.

Startup Configuration File
The startup configuration file is the saved configuration file that sets the properties of the device each time the device is powered up. This file is stored in non-volatile RAM (NVRAM), meaning that it is saved even when power to the device is turned off.
When a Cisco router is first powered up, it loads the Cisco IOS software to working memory, or RAM. Next, the startup configuration file is copied from NVRAM to RAM. When the startup configuration file is loaded into RAM, the file becomes the initial running configuration.

Running Configuration File
The term running configuration refers to the current configuration running in RAM on the device. This file contains the commands used to determine how the device operates on the network.
The running configuration file is stored in the working memory of the device. Changes to the configuration and various device parameters can be made when the file is in working memory. However, the running configuration is lost each time the device is shut down, unless the running configuration is saved to the startup configuration file.

Changes to the running configuration are not automatically saved to the startup configuration file. It is necessary to manually copy the running configuration to the startup configuration file.
When configuring a device via the Cisco command line interface (CLI) the command copy running-config startup-config, or the abbreviated version copy run start, saves the running configuration to the startup configuration file. When configuring a device via the Cisco SDM GUI, there is an option to save the router running configuration to the startup configuration file each time a command is completed.

Bootup Process

After the startup configuration file is loaded and the router boots successfully, the show version command can be used to verify and troubleshoot some of the basic hardware and software components used during the bootup process. The output from the show version command includes:
– The Cisco IOS software version being used.
– The version of the system bootstrap software, stored in ROM memory, that was initially used to boot the router.
– The complete filename of the Cisco IOS image and where the bootstrap program located it.
– Type of CPU on the router and amount of RAM. It may be necessary to upgrade the amount of RAM when upgrading the Cisco IOS software.
– The number and type of physical interfaces on the router.
– The amount of NVRAM. NVRAM is used to store the startup-config file.
– The amount of flash memory on the router. Flash is used to permanently store the Cisco IOS image. It may be necessary to upgrade the amount of flash when upgrading the Cisco IOS software.
– The current configured value of the software configuration register in hexadecimal.

The configuration register tells the router how to boot up. For example, the factory default setting for the configuration register is 0x2102. This value indicates that the router attempts to load a Cisco IOS software image from flash and loads the startup configuration file from NVRAM. It is possible to change the configuration register and, therefore, change where the router looks for the Cisco IOS image and the startup configuration file during the bootup process. If there is a second value in parentheses, it denotes the configuration register value to be used during the next reload of the router.

There are times when the router does not successfully boot. This failure can be caused by a number of factors, including a corrupt or missing Cisco IOS file, an incorrect location for the Cisco IOS image specified by the configuration register, or inadequate memory to load a new Cisco IOS image. If the router fails to boot the IOS, it then boots up in ROM monitor (ROMmon) mode. ROMmon software is a simple command set stored in read only memory (ROM) that can be used to troubleshoot boot errors and recover the router when the IOS is not present.

When the router boots up to ROMmon mode, one of the first steps in troubleshooting is to look in flash memory for a valid image using the dir flash: command. If an image is located, attempt to boot the image with the boot flash: command.

rommon 1>boot flash:c2600-is-mz.121-5

If the router boots properly with this command, there are two possible reasons why the Cisco IOS image did not load from flash initially. First, use the show version command to check the configuration register to ensure that it is configured for the default boot sequence. If the configuration register value is correct, use the show startup-config command to see if there is a boot system command that instructs the router to use a different location for the Cisco IOS image.

Cisco ISO Programs

There are two methods to connect a PC to a network device to perform configuration and monitoring tasks: out-of-band management and in-band management.

Out-of-band Management
Out-of-band management requires a computer to be directly connected to the console port or auxiliary port (AUX) of the network device being configured. This type of connection does not require the local network connections on the device to be active. Technicians use out-of-band management to initially configure a network device, because until properly configured, the device cannot participate in the network. Out-of-band management is also useful when the network connectivity is not functioning correctly and the device cannot be reached over the network. Performing out-of-band management tasks requires a terminal emulation client installed on the PC.

In-band Management
Use in-band management to monitor and make configuration changes to a network device over a network connection. For a computer to connect to the device and perform in-band management tasks, at least one network interface on the device must be connected to the network and be operational. Either Telnet, HTTP or SSH can be used to access a Cisco device for in-band management. A web browser or a Telnet client program can be used to monitor the network device or make configuration changes.

The Cisco IOS command line interface (CLI) is a text-based program that enables entering and executing Cisco IOS commands to configure, monitor, and maintain Cisco devices. The Cisco CLI can be used with either in-band or out-of-band management tasks.

Use CLI commands to alter the configuration of the device and to display the current status of processes on the router. For experienced users, the CLI offers many time-saving features for creating both simple and complex configurations. Almost all Cisco networking devices use a similar CLI. When the router has completed the power-up sequence, and the Router> prompt appears, the CLI can be used to enter Cisco IOS commands.

Technicians familiar with the commands and operation of the CLI find it easy to monitor and configure a variety of different networking devices. The CLI has an extensive help system that assists users in setting up and monitoring devices.

In addition to the Cisco IOS CLI, other tools are available to assist in configuring a Cisco router or ISR. Security Device Manager (SDM) is a web-based GUI device management tool. Unlike CLI, SDM can be used only for in-band management tasks.

SDM Express simplifies the initial router configuration. It uses a step-by-step approach to create a basic router configuration quickly and easily.

The full SDM package offers more advanced options, such as:
– Configuring additional LAN and WAN connections
– Creating firewalls
– Configuring VPN connections
– Performing security tasks

SDM supports a wide range of Cisco IOS software releases and is available free of charge on many Cisco routers. SDM is pre-installed on the flash memory of the Cisco 1800 Series ISR. If the router has SDM installed, it is good practice to use SDM to perform the initial router configuration. This configuration is done by connecting to the router via a preset network port on the router.

Not all Cisco devices support SDM. In addition, SDM does not support all the commands that are available through the CLI. Consequently, it is sometimes necessary to use the CLI to complete a device configuration that is started using SDM. Familiarity with both methods is critical to successfully support Cisco devices.

Cisco SDM Express

When adding a new device to a network, it is critical to ensure that the device functions correctly. The addition of one poorly configured device can cause an entire network to fail.

Configuring a networking device, such as a router, can be a complex task, no matter which tool is used to enter the configuration. Therefore, follow best practices for installing a new device to ensure that all device settings are properly configured and documented.

Cisco SDM Express is a tool bundled within the Cisco Router and Security Device Manager that makes it easy to create a basic router configuration. To start using SDM Express, connect an Ethernet cable from the PC NIC to the Ethernet port specified in the quick start guide on the router or ISR being configured.

SDM Express uses eight configuration screens to assist in creating a basic router configuration:

– Overview
– Basic Configuration
– LAN IP Address
– Internet (WAN)
– Firewall
– Security Settings
– Summary

The SDM Express GUI provides step-by-step guidance to create the initial configuration of the router. After the initial configuration is completed, the router is available on the LAN. The router can also have a WAN connection, a firewall, and up to 30 security enhancements configured.

The SDM Express Basic Configuration screen contains basic settings for the router that is being configured. The following information is required:
Host name – The name assigned to the router being configured.
Domain name for the organization – An example of a domain name is, but domain names can end with a different suffix, such as .org or .net.
Username and password – The username and password used to access SDM Express to configure and monitor the router. The password must be at least six characters long.
Enable secret password – The password that controls user access to the router, which affects the ability to make configuration changes using the CLI , Telnet, or the console ports. The password must be at least six characters long.

The LAN configuration settings enable the router interface to participate on the connected local network.
IP address – Address for the LAN interface in dotted-decimal format. It can be a private IP address if the device is installed in a network that uses Network Address Translation (NAT) or Port Address Translation (PAT).

It is important to take note of this address. When the router is restarted, this address is the one used to access SDM Express, not the address that was provided in the Quick Start guide.
Subnet mask – Identifies the network portion of the IP address.
Subnet bits – Number of bits used to define the network portion of the IP address. The number of bits can be used instead of the subnet mask.
Wireless parameters – Optional. Appear if the router has a wireless interface, and Yes was clicked in the Wireless Interface Configuration window. Specifies the SSID of the wireless network.

DHCP is a simple way to assign IP addresses to host devices. DHCP dynamically allocates an IP address to a network host when the host is powered up, and reclaims the address when the host is powered down. In this way, addresses can be reused when hosts no longer need them. Using SDM Express, a router can be configured as a DHCP server to assign addresses to devices, such as PCs, on the internal local network.

To configure a device for DHCP, select the Enable DHCP Server on the LAN Interface checkbox. Checking this box enables the router to assign private IP addresses to devices on the LAN. IP addresses are leased to hosts for a period of one day.

DHCP uses a range of allowable IP addresses. By default, the valid address range is based on the IP address and subnet mask entered for the LAN interface.
The starting address is the lowest address in the IP address range. The starting IP address can be changed, but it must be in the same network or subnet as the LAN interface.
The ending IP address is the highest address in the IP address range and it can be changed to decrease the pool size. It must be in the same network as the IP starting address.

Additional DHCP configuration parameters include:
Domain name for the organization – This name is given to the hosts as part of the DHCP configuration.
Primary domain name server – IP address of the primary DNS server. Used to resolve URLs and names on the network.
Secondary domain name server – IP address of a secondary DNS sever, if available. Used if the primary DNS server does not respond.

Selecting Use these DNS values for DHCP clients enables the DHCP server to assign DHCP clients with the configured DNS settings. This option is available if a DHCP server has been enabled on the LAN interface.

Configuring an Internet (WAN) Connection

A serial connection can be used to connect networks that are separated by large geographic distances. These WAN network interconnections require a telecommunications service provider (TSP).

Serial connections are usually lower speed links, compared to Ethernet links, and require additional configuration. Prior to setting up the connection, determine the type of connection and protocol encapsulation required.

The protocol encapsulation must be the same at both ends of a serial connection. Some encapsulation types require authentication parameters, like username and password, to be configured. Encapsulation types include:
– High-Level Data Link Control (HDLC)
– Frame Relay
– Point-to-Point Protocol (PPP)

The WAN configuration window has additional WAN parameters.

Address Type List

Depending on the type of encapsulation selected, different methods of obtaining an IP address for the serial interface are available:
Static IP address – Available with Frame Relay, PPP, and HDLC encapsulation types. To configure a static IP address, enter the IP address and subnet mask.
IP unnumbered – Sets the serial interface address to match the IP address of one of the other functional interfaces of the router. Available with Frame Relay, PPP, and HDLC encapsulation types.
IP negotiated – The router obtains an IP address automatically through PPP.
Easy IP (IP Negotiated) – The router obtains an IP address automatically through PPP.

Either Cisco SDM Express or Cisco SDM can be used to configure a router.
SDM supports many of the same features that SDM Express supports; however, SDM has more advanced configuration options. For this reason, after the router basic configuration is completed using SDM Express, many users switch to SDM. For example, enabling NAT requires the use of SDM.

The Basic NAT Wizard configures Dynamic NAT with PAT, by default. PAT enables the hosts on the internal local network to share the single registered IP address assigned to the WAN interface. In this manner, hosts with internal private addresses can have access to the Internet.
Only the hosts with the internal address ranges specified in the SDM configuration are translated. It is important to verify that all address ranges that need access to the Internet are included.

Steps for configuring NAT include:
Step 1. Enable NAT configuration using SDM.
Step 2. Navigate through the Basic NAT Wizard.
Step 3. Select the interface and set IP ranges.
Step 4. Review the configuration.

Command Line Interface Modes

Using the Cisco IOS CLI to configure and monitor a device is very different from using SDM. The CLI does not provide step-by-step configuration assistance; therefore, it requires more planning and expertise to use.

CLI Command Modes
The Cisco IOS supports two levels of access to the CLI: user EXEC mode and privileged EXEC mode.
When a router or other Cisco IOS device is powered up, the access level defaults to user EXEC mode. This mode is indicated by the command line prompt:

Commands that can be executed in user EXEC mode are limited to obtaining information about how the device is operating, and troubleshooting using some show commands and the ping and traceroute utilities.
To enter commands that can alter the operation of the device requires privileged level access. Enable the privileged EXEC mode by entering enable at the command prompt and pressing Enter.
The command line prompt changes to reflect the mode change. The prompt for privileged EXEC mode is:

To disable the privileged mode and return to user mode, enter disable at the command prompt.
Both modes can be protected with a password, or a username and password combination.

Various configuration modes are used to set up a device. Configuring a Cisco IOS device begins with entering privileged EXEC mode. From privileged EXEC mode, the user can access the other configuration modes.

In most cases, commands are applied to the running configuration file using a terminal connection. To use these commands, the user must enter global configuration mode.

To enter global configuration, type the command configure terminal or config t. Global configuration mode is indicated by the command line prompt:

Any commands entered in this mode take effect immediately and can alter the operation of the device.
From global configuration mode, the administrator can enter other sub-modes.

Interface configuration mode is used to configure LAN and WAN interfaces. To access interface configuration mode, from global configuration type the command interface [type] [number]. Interface configuration mode is indicated by the command prompt:

Another commonly used sub-mode is the router configuration submode represented by the following prompt:

This mode is used to configure routing parameters.

Using the Cisco IOS CLI

The Cisco IOS CLI is full of features that help in recalling commands needed to configure a device. These features are one reason why network technicians prefer to use the Cisco IOS CLI to configure routers.

The context-sensitive help feature is especially useful when configuring a device. Entering help or the ? at the command prompt displays a brief description of the help system.

Router# help

Context-sensitive help can provide suggestions for completing a command. If the first few characters of a command are known but the exact command is not, enter as much of the command as possible, followed by a ?. Note that there is no space between the command characters and the ?.

Additionally, to get a list of the parameter options for a specific command, enter part of the command, followed by a space, and then the ?. For example, entering the command configure followed by a space and a ? shows a list of the possible variations. Choose one of the entries to complete the command string. Once the command string is completed, a <cr> appears. Press Enter to issue the command.

If a ? is entered and nothing matches, the help list will be empty. This indicates that the command string is not a supported command.

Users sometimes make a mistake when typing a command. The CLI indicates if an unrecognized or incomplete command is entered. The % symbol marks the beginning of an error message. For example, if the command interface is entered with no other parameters, an error message displays indicating an incomplete command:
% Incomplete command

Use the ? to get a list of the available parameters.

If an incorrect command is entered, the error message would read:
% Invalid input detected

It is sometimes hard to see the mistake within an incorrectly entered command. Fortunately, the CLI provides an error indicator. The caret symbol (^) appears at the point in the command string where there is an incorrect or unrecognized character. The user can return to the point where the error was made and use the help function to determine the correct command to use.

Another feature of the Cisco IOS CLI is the ability to recall previously typed commands. This feature is particularly useful for recalling long or complex commands or entries.

The command history is enabled by default and the system records 10 command lines in the history buffer. To change the number of command lines the system records during a session, use the terminal history size or the history size command. The maximum number of command lines is 256.
To recall the most recent command in the history buffer, press Ctrl-P or the Up Arrow key. Repeat this process to recall successively older commands. To return to a more recent command in the history buffer, press Ctrl-N or the Down Arrow key. Repeat this process to recall successively more recent commands.
The CLI recognizes partially typed commands based on their first unique character. For example, type int instead of interface. If a short cut, such as int is entered, pressing the Tab key will automatically complete the entire command entry of interface.
On most computers, additional select and copy functions are available using various function keys. A previous command string may be copied and then pasted or inserted as the current command entry.

Using Show Commands

The Cisco IOS CLI includes show commands that display relevant information about the configuration and operation of the device.

Network technicians use the show commands extensively for viewing configuration files, checking the status of device interfaces and processes, and verifying the device operational status. Show commands are available whether the device was configured using the CLI or SDM.

The status of nearly every process or function of the router can be displayed using a show command. Some of the more popular show commands are:
show running-config
show interfaces
show arp
show ip route
show protocols
show version

Basic Configuration

The initial configuration of a Cisco IOS device involves configuring the device name and then the passwords that are used to control access to the various functions of the device.

A device should be given a unique name as one of the first configuration tasks. This task is accomplished in global configuration mode with the following command.

Router(config)# hostname [name]

When the Enter key is pressed, the prompt changes from the default host name, which is Router, to the newly configured host name.

The next configuration step is to configure passwords to prevent access to the device by unauthorized individuals.

The enable password and enable secret commands are used to restrict access to privileged EXEC mode, preventing unauthorized users from making configuration changes to the router.

Router(config)# enable password [password]

Router(config)# enable secret [password]

The difference between the two commands is that the enable password is not encrypted by default. If the enable password is set, followed by the enable secret password, the enable secret command overrides the enable password command.

Other basic configurations of a router include configuring a banner, enabling synchronous logging, and disabling domain lookup.

A banner is text that a user sees when initially logging on to the router. Configuring an appropriate banner is part of a good security plan. At a very minimum, a banner should warn against unauthorized access. Never configure a banner that welcomes an unauthorized user.
There are two types of banners: message-of-the-day (MOTD) and login information. The purpose for two separate banners is to be able to change one without affecting the entire banner message.
To configure the banners, the commands are banner motd and banner login. For both types, a delimiting character, such as a #, is used at the beginning and at the end of the message. The delimiter allows the user to configure a multiline banner.
If both banners are configured, the login banner appears after the MOTD but before the login credentials.

Synchronous Logging
The Cisco IOS software often sends unsolicited messages, such as a change in the state of a configured interface. Sometimes these messages occur in the middle of typing a command. The message does not affect the command, but can cause the user confusion when typing. To keep the unsolicited output separate from the typed input, the logging synchronous command can be entered in global configuration mode.

Disabling Domain Lookup
By default, when a host name is entered in enable mode, the router assumes that the user is attempting to telnet to a device. The router tries to resolve unknown names entered in enable mode by sending them to the DNS server. This process includes any words entered that the router does not recognize, including mistyped commands. If this capability is not wanted, the no ip domain-lookup command turns off this default feature.

There are multiple ways to access a device to perform configuration tasks. One of these ways is to use a PC attached to the console port on the device. This type of connection is frequently used for initial device configuration.

Setting a password for console connection access is done in global configuration mode. These commands prevent unauthorized users from accessing user mode from the console port.

Route(config)# line console 0

Router(config)# password [password]

Router(config)# login

When the device is connected to the network, it can be accessed over the network connection. When the device is accessed through the network, it is considered a vty connection. The password must be configured on the vty port.

Route(config)# line vty 0 4

Router(config)# password [password]

Router(config)# login

0 4 represents 5 simultaneous in-band connections. It is possible to set a different password for each connection by specifing specific line connection numbers, such as line vty 0.

To verify that the passwords are set correctly, use the show running-config command. These passwords are stored in the running-configuration in clear text. It is possible to set encryption on all passwords stored within the router so that they are not easily read by unauthorized individuals. The global configuration command service password-encryption ensures that all passwords are encrypted.

Remember, if the running configuration is changed, it must be copied to the startup configuration file or the changes are lost when the device is powered down. To copy the changes made to the running configuration back to the stored startup configuration file, use the copy run start command.

Configuring an Interface

To direct traffic from one network to another, router interfaces are configured to participate in each of the networks. A router interface connecting to a network will typically have an IP address and subnet mask assigned that is within the host range for the connected network.

There are different types of interfaces on a router. Serial and Ethernet interfaces are the most common. Local network connections use Ethernet interfaces.

WAN connections require a serial connection through an ISP. Unlike Ethernet interfaces, serial interfaces require a clock signal to control the timing of the communications, called a clock rate. In most environments, data communications equipment (DCE) devices, such as a modem or CSU/DSU, provide the clock rate.

When a router connects to the ISP network using a serial connection, a CSU/DSU is required if the WAN is digital. A modem is required if the WAN is analog. These devices convert the data from the router into a form acceptable for crossing the WAN, and convert data from the WAN into an acceptable format for the router. By default, Cisco routers are data terminal equipment (DTE) devices. Because the DCE devices control the timing of the communication with the router, the Cisco DTE devices accept the clock rate from the DCE device.

Though uncommon, it is possible to connect two routers directly together using a serial connection. In this instance, no CSU/DSU or modem is used, and one of the routers must be configured as a DCE device to provide clocking. If the router is connected as the DCE device, a clock rate must be set on the router interface to control the timing of the DCE/DTE connection.

Configuring an interface on the router must be done in global configuration mode. Configuring an Ethernet interface is very similar to configuring a serial interface. One of the main differences is that a serial interface must have a clock rate set if it is acting as a DCE device.

The steps to configure an interface include:
Step 1. Specify the type of interface and the interface port number.
Step 2. Specify a description of the interface.
Step 3. Configure the interface IP address and subnet mask.
Step 4. Set the clock rate, if configuring a serial interface as a DCE.
Step 5. Enable the interface.

After an interface is enabled, it may be necessary to turn off an interface for maintenance or troubleshooting. In this case, use the shutdown command.

When configuring the serial interface on a 1841, the serial interface is designated by 3 digits, C/S/P, where C=Controller#, S=Slot# and P=Port#. The 1841 has two modular slots. The designation Serial0/0/0 indicates that the serial interface module is on controller 0, in slot 0, and that the interface to be used is the first one (0). The second interface is Serial0/0/1. The serial module is normally installed in slot 0 but may be installed in slot 1. If this is the case, the designation for the first serial interface would be Serial0/1/0 and the second would be Serial0/1/1.

For built in ports, such as the FastEthernet ports the designation is 2 digits, C/P, where C=Controller#, and P=Port#. The designation Fa0/0 represents controller 0 and interface 0.

Configuring a Default Route

A router forwards packets from one network to another based on the destination IP address specified in the packet. It examines the routing table to determine where to forward the packet to reach the destination network. If the router does not have a route to a specific network in its routing table, a default route can be configured to tell the router how to forward the packet. The default route is used by the router only if the router does not know where to send a packet.

Usually, the default route points to the next hop router on the path to the Internet. The information needed to configure the default route is the IP address of the next hop router, or the interface that the router uses to forward traffic with an unknown destination network.

Configuring the default route on a Cisco ISR must be done in global configuration mode.

Router(config)# ip route [next-hop-IP-address]
Router(config)# ip route [interface-type] [number]

Configuring DHCP Services

The Cisco IOS CLI can be used to configure a router to function as a DHCP server.
Using a router configured with DHCP simplifies the management of IP addresses on a network. The administrator needs to update only a single, central router when IP configuration parameters change. Configuring DHCP using the CLI is a little more complex than configuring it using SDM.

There are eight basic steps to configuring DHCP using the CLI.
Step 1. Create a DHCP address pool.
Step 2. Specify the network or subnet.
Step 3. Exclude specific IP addresses.
Step 4. Specify the domain name.
Step 5. Specify the IP address of the DNS server.
Step 6. Set the default gateway.
Step 7. Set the lease duration.
Step 8. Verify the configuration.

Configuring Static NAT Using Cisco IOS

NAT enables hosts with internal private addresses to communicate on the Internet. When configuring NAT, at least one interface must be configured as the inside interface. The inside interface is connected to the internal, private network. Another interface, usually the external interface used to access the Internet, must be configured as the outside interface. When devices on the internal network communicate out through the external interface, the addresses are translated to one or more registered IP addresses.

There are occasions when a server located on an internal network must be accessible from the Internet. This accessibility requires that the server has a specific registered address that external users can specify. One way to provide this address to an internal server is to configure a static translation.

Static NAT ensures that addresses assigned to hosts on the internal network are always translated to the same registered IP address.

Configuring NAT and static NAT using the Cisco IOS CLI requires a number of steps.
Step 1. Specify the inside interface.
Step 2. Set the primary IP address of the inside interface.
Step 3. Identify the inside interface using the ip nat inside command.
Step 4. Specify the outside interface.
Step 5. Set the primary IP address of the outside interface.
Step 6. Identify the outside interface using the ip nat outside command.
Step 7. Define the static address translation.
Step 8. Verify the configuration.

There are several router CLI commands to view NAT operations for verification and troubleshooting.

One of the most useful commands is show ip nat translations. The output displays the detailed NAT assignments. The command shows all static translations that have been configured and any dynamic translations that have been created by traffic. Each translation is identified by protocol and its inside and outside local and global addresses.

The show ip nat statistics command displays information about the total number of active translations, NAT configuration parameters, how many addresses are in the pool, and how many have been allocated.

Additionally, use the show run command to view NAT configurations.

By default, if dynamic NAT is configured, translation entries time out after 24 hours. It is sometimes useful to clear the dynamic entries sooner than 24 hours. This is especially true when testing the NAT configuration. To clear dynamic entries before the timeout has expired, use the clear ip nat translation * command in the enable mode. Only the dynamic translations are removed from the table. Static translations cannot be cleared from the translation table.

Backing Up a Cisco Router Configuration

After a router is configured, the running configuration should be saved to the startup configuration file. It is also a good idea to save the configuration file in another location, such as a network server. If the NVRAM fails or becomes corrupt and the router cannot load the startup configuration file, another copy is available. There are multiple ways that a configuration file can be saved.

One way configuration files can be saved to a network server is using TFTP. The TFTP server must be accessible to the router via a network connection.
Step 1. Enter the copy startup-config tftp command.
Step 2. Enter the IP address of the host where the configuration file will be stored.
Step 3. Enter the name to assign to the configuration file or accept the default.
Step 4. Confirm each choice by answering yes.

The running configuration can also be stored on a TFTP server using the copy running-config tftp command.

To restore the backup configuration file, the router must have at least one interface configured and be able to access the TFTP server over the network.
Step 1. Enter the copy tftp running-config command.
Step 2. Enter the IP address of the remote host where the TFTP server is located.
Step 3. Enter the name of the configuration file or accept the default name.
Step 4. Confirm the configuration filename and the TFTP server address.
Step 5. Using the copy run start command, copy the running-configuration to the startup-configuration file to ensure that the restored configuration is saved.

When restoring your configuration, it is possible to copy the tftp file to the startup configuration file. However, this does require a router reboot in order to load the startup configuration file into the running configuration.

Another way to create a backup copy of the configuration is to capture the output of the show running-config command. To do this from the terminal session, copy the output, paste it into a text file, and then save the text file.

The following steps are used to capture the configuration from a HyperTerminal screen.
Step 1. Select Transfer.
Step 2. Select Capture Text.
Step 3. Specify a name for the text file to capture the configuration.
Step 4. Select Start to start capturing text.
Step 5. Use the show running-config command to display the configuration on the screen.
Step 6. Press the spacebar when each “-More -” prompt appears.

After the complete configuration has been displayed, the following steps stop the capture.
Step 1. Select Transfer.
Step 2. Select Capture Text.
Step 3. Select Stop.

After the capture is complete, the configuration file must be edited to remove extra text, such as the “building configuration” Cisco IOS message. Also, the no shutdown command must be added to the end of each interface section. Click File > Save to save the configuration. The configuration file can be edited from a text editor such as Notepad.

The backup configuration can be restored from a HyperTerminal session. Before the configuration is restored, any other configurations should be removed from the router using the erase startup-config command at the privileged EXEC prompt. The router is then restarted using the reload command.

The following steps copy the backup configuration to the router.
Step 1. Enter router global configuration mode.
Step 2. Select Transfer > Send Text File in HyperTerminal.
Step 3. Select the name of the file for the saved backup configuration.
Step 4. Restore the startup configuration with the copy run start command

Installing the CPE

One of the main responsibilities of an on-site network technician is to install and upgrade equipment located at a customer home or business. Network devices installed at the customer location are called customer premises equipment (CPE) and include devices such as routers, modems, and switches.

The installation or upgrade of a router can be disruptive for a business. Many businesses rely on the Internet for their correspondence and have e-commerce services that must be accessed during the day. Planning the installation or upgrade is a critical step in ensuring successful operation. Additionally, planning enables options to be explored on paper, where it is easy and inexpensive to correct errors.

The ISP technical staff usually meets with business customers for planning. During planning sessions, the technician determines the configuration of the router to meet customer needs and the network software that may be affected by the new installation or upgrade.

The technician works with the IT personnel of the customer to decide which router configuration to use and to develop the procedure that verifies the router configuration. From this information, the technician completes a configuration checklist.

The configuration checklist provides a list of the most commonly configured components. It typically includes an explanation of each component and the configuration setting. The list is a tool for ensuring that everything is configured correctly on new router installations. It is also helpful for troubleshooting previously configured routers.

There are many different formats for configuration checklists, including some that are quite complex. ISPs should ensure that support technicians have, and know how to use, router configuration checklists.

When new equipment is required, the devices are typically configured and tested at the ISP site before being installed at the customer site. Anything that is not functioning as expected can be replaced or fixed immediately. If a router is being installed, the network technician makes sure that the router is fully configured and that the router configuration is verified.

When the router is known to be configured correctly, all network cables, power cables, management cables, manufacturer documentation, manufacturer software, configuration documentation, and the special tools needed for router installation are assembled. An inventory checklist is used to verify that all necessary equipment needed to install the router is present. Usually, the network technician signs the checklist, indicating that everything has been verified. The signed and dated inventory checklist is included with the router when it is packaged for shipping to the customer premises.

The router is now ready to be installed by the on-site technician. It is important to find a time that provides the minimum amount of disruption. It may not be possible to install or upgrade network equipment during normal business hours. If the installation will cause the network to be down, the network technician, the ISP sales person, and a representative of the company prepare a router installation plan. This plan ensures that the customer experiences a minimum of disruption in service while the new equipment is installed. Additionally, the router installation plan identifies who the customer contact is and what the arrangements are for access to the site after business hours. As part of the installation plan, an installation checklist is created to ensure that equipment is installed appropriately.

The on-site network technician must install the router at the customer premises using the router installation plan and checklist. When installing customer equipment, it is important to complete the job in a professional manner. This means that all network cables are labeled and fastened together or run through proper cable management equipment. Excess lengths of cable are coiled and secured out of the way.

Documentation should be updated to include the current configuration of the router, and network diagrams should be updated to show the location of the equipment and cables installed.

After the router is successfully installed and tested, the network technician completes the installation checklist. The completed checklist is then verified by the customer representative. The verification of the router installation often involves demonstrating that the router is correctly configured and that services that depend on the router work as expected.

When the customer representative is satisfied that the router has been correctly installed and is operational, the customer signs and dates the checklist. Sometimes there is a formal acceptance document in addition to the checklist. This procedure is often called the sign-off phase. It is critical that the customer representative signs off on the job, because the ISP can then bill the customer for the work.

Installation Documentation

When customer equipment is configured and installed on the customer premises, it is important to document the entire process. Documentation includes all aspects of equipment configuration, diagrams of equipment installation, and checklists to validate the correct installation. If a new configuration is needed, the documentation is compared with the previous router configuration to determine if and how the new configuration has changed. Activity logs are used to track modifications and access to equipment. Properly maintained activity logs help when troubleshooting problems.

The technician starts documenting the work during router installation. All cables and equipment are correctly labeled and indicated on a diagram to simplify future identification.

The technician uses the installation and verification checklist when installing a router. This checklist displays the tasks to be completed at the customer premises. The checklist helps the network technician avoid errors and ensures that the installation is done efficiently and correctly.

A copy of the final documentation is left with the customer.

Customer Connections over a WAN

New equipment at the customer site must be connected back to the ISP to provide Internet services. When customer equipment is upgraded, it is sometimes necessary to also upgrade the type of connectivity provided by the ISP.

Wide Area Networks
When a company or organization has locations that are separated by large geographical distances, it may be necessary to use the telecommunications service provider (TSP) to interconnect the LANs at the different locations. The networks that connect LANs in geographically separated locations are referred to as wide area networks (WANs).

TSPs operate large regional networks that can span long distances. Traditionally, TSPs transported voice and data communications on separate networks. Increasingly, these providers are offering converged information network services to their subscribers.

Individual organizations usually lease connections through the TSP network. Although the organization maintains all the policies and administration of the LANs at both ends of the connection, the policies within the communications service provider network are controlled by the ISP.

ISPs sell various types of WAN connections to their clients. WAN connections vary in the type of connector used, in bandwidth, and in cost. As small businesses grow, they require the increased bandwidth offered by some of the more expensive WAN connections. One of the jobs at an ISP or medium-sized business is to assess what type of WAN connection is needed.

There are three types of serial WAN connections.

A point-to-point connection is a predefined communications path from the customer premises through a TSP network. It is a dedicated circuit with fixed bandwidth available at all time. Point-to-point lines are usually leased from the TSP. These lines are often called leased lines. Point-to-point connections are typically the most expensive of the WAN connection types, and are priced based on the bandwidth required and the distance between the two connected points. An example of a point-to-point WAN connection is a T1 or E1 link.

A circuit-switched connection functions similarly to the way a phone call is made over a telephone network. When making a phone call to a friend, the caller picks up the phone, opens the circuit, and dials the number. The caller hangs up the phone when finished and the closes the circuit. An example of a circuit-switched WAN connection is an ISDN or dialup connection.

In a packet-switched connection, networks have connections into the TSP switched network. Many customers share this TSP network. Instead of the circuit being physically reserved from source to destination, as in a circuit-switched network, each customer has its own virtual circuit. A virtual circuit is a logical path between the sender and receiver, not a physical path. An example of a packet-switched network is Frame Relay.

Choosing a WAN Connection

When choosing a WAN, the decision is largely dependent on the bandwidth and cost of the WAN connection. Smaller businesses are not able to afford some of the more expensive WAN connection options, such as SONET or ATM WAN connections. They usually install the less expensive DSL, cable, and T1 connections. In addition, higher bandwidth WAN connections may not be available in geographically isolated locations. If the offices supported are close to an urban center, there are more WAN choices.

Another factor that affects the decision on which WAN to choose is how the business plans to use the connection. If the business provides services over the Internet, it may require higher upstream bandwidth. For example, if a business hosts a web server for an e-commerce business, it needs enough upstream bandwidth to accommodate the number of external customers that visit its site. On the other hand, if the business uses an ISP to manage its e-commerce site, the business does not need as much upstream bandwidth.

For some businesses, the ability to get a service level agreement (SLA) with their WAN connection affects their decision. Less expensive WAN connections like dialup, DSL, and cable typically do not come with an SLA, whereas more expensive connections do.

There are many things to consider when planning a WAN upgrade. The ISP initiates the process by analyzing the customer needs and reviewing the available options. A proposal is then generated for the customer. The proposal addresses the existing infrastructure, the customer requirements, and possible WAN options.

Existing Infrastructure
This is an explanation of the current infrastructure being used by the business. It helps the customer understand how the existing WAN connection provides services to their home or business.

Customer Requirements
This section of the proposal describes why a WAN upgrade is necessary for the customer. It outlines where the current WAN connection does not meet the customer needs. It also includes a list of requirements that the new WAN connection must meet to satisfy the current and future customer requirements.

WAN Options
This is a list of all the available WAN choices with the corresponding bandwidth, cost, and other features that are applicable for the business is included in the proposal. The recommended choice is indicated, including possible other options.

The WAN upgrade proposal is presented to the business decision-makers. They review the document and consider the options. When they have made their decision, the ISP works with the customer to develop a schedule and coordinate the WAN upgrade process.

Configuring WAN Connections

How a WAN is configured depends on the type of WAN connection required. Some WAN connections support Ethernet interfaces. Other WAN connections support serial interfaces.

Leased-line WAN connections typically use a serial connection, and require a channel service unit and data service unit (CSU/DSU) to attach to the ISP network. The ISP equipment needs to be configured so that it can communicate through the CSU/DSU to the customer premises.

For a serial connection, it is important to have a preconfigured clock rate that is the same on both ends of the connection. The clock rate is set by the DCE device, which is typically the CSU/DSU. The DTE device, typically the router, accepts the clock rate set by the DCE.

The Cisco default serial encapsulation is HDLC. It can be changed to PPP, which provides a more flexible encapsulation and supports authentication by the remote device.

Standalone Switches

Although the integrated swith module of the 1841 ISR is adequate for connecting a small number of hosts to the LAN, it may be necessary to add larger, more capable switches to support additional users as the network grows.

A switch is a device that directs a stream of messages from one port to another based on the destination MAC address within the frame. A switch cannot route traffic between two different local networks. In the context of the OSI model, a switch performs Layer 2 functions. Layer 2 is the Data Link Layer.

Several models of Ethernet switches are available to meet various user requirements. The Cisco Catalyst 2960 Series Ethernet switch is designed for the networks of medium-sized businesses and branch offices.

The Catalyst 2960 Series of switch are fixed-configuration, standalone devices that do not support modules or flash card slots. Because the physical configuration cannot change, fixed-configuration switches must be chosen based on the required number and type of ports. 2960 Series switches can provide 10/100 Fast Ethernet and 10/100/1000 Gigabit Ethernet connectivity. These switches use Cisco IOS software and can be configured using a GUI-based Cisco Network Assistant or through the CLI.

All switches support both half-duplex or full-duplex mode.

When a port is in half-duplex mode, at any given time, it can either send or receive data but not both. When a port is in full-duplex mode, it can simultaneously send and receive data, doubling the throughput.

Both the port and the connected device must be set to the same duplex mode. If they are not the same, a duplex mismatch occurs, which can lead to excessive collisions and degraded communication.

The speed and duplex can be set manually, or the switch port can use autonegotiation. Autonegotiation allows the switch to autodetect the speed and duplex of the device that is connected to the port. Autonegotiation is enabled by default on many Cisco switches.

For autonegotiation to be successful, both devices must support it. If the switch is in autonegotiation mode and the connected device does not support it, the switch uses the speed of the other device (10, 100, or 1000) and is set to half-duplex mode. Defaulting to half duplex can create problems if the non-autonegotiating device is set to full duplex.

If the connected device does not autonegotiate, manually configure the duplex settings on the switch to match the duplex settings on the connected device. The speed parameter can adjust itself, even if the connected port does not autonegotiate.

Switch settings, including the speed and duplex port parameters, can be configured using the Cisco IOS CLI. When configuring a switch using the Cisco IOS CLI, the interface and command structure is very similar to the Cisco routers.

As with the Cisco routers, there is a variety of choices for the Cisco IOS image for switches. The IP-base software image is supplied with the Cisco Catalyst 2960 switch. This image provides the switch with basic switching capabilities and IP services. Other Cisco IOS software images supply additional services to the IP-base image.

Powering Up the Cisco 2960 Switch

Powering up a Cisco 2960 switch is similar to powering up a Cisco 1841 ISR.

The three basic steps for powering up a switch include:
Step 1. Check the components.
Step 2. Connect the cables to the switch.
Step 3. Power up the switch.

When the switch is on, the power-on self-test (POST) begins. During POST, the LEDs blink while a series of tests determine that the switch is functioning properly.

POST is completed when the SYST LED rapidly blinks green. If the switch fails POST, the SYST LED turns amber. When a switch fails POST, it is necessary to return the switch for repairs.

When all startup procedures are finished, the Cisco 2960 switch is ready to configure.

Initial Switch Configuration

There are several ways to configure and manage a Cisco LAN switch.
– Cisco Network Assistant
– Cisco Device Manager
– Cisco IOS CLI
– CiscoView Management Software
– SNMP Network Management Products

Some of these methods use IP connectivity or a web browser to connect to the switch, which requires an IP address. Unlike router interfaces, switch ports are not assigned IP addresses. To use an IP-based management product or Telnet session to manage a Cisco switch, it is necessary to configure a management IP address on the switch.

If the switch does not have an IP address, it is necessary to connect directly to the console port and use a terminal emulation program to perform configuration tasks.

he Cisco Catalyst 2960 switch comes preconfigured and only needs to be assigned basic security information before being connected to the network.

The commands to configure the host name and passwords on the switch are the same commands used to configure the ISR. To use an IP-based management product or Telnet with a Cisco switch, configure a management IP address.

To assign an address to a switch, the address must be assigned to a virtual local area network VLAN interface. A VLAN allows multiple physical ports to be grouped together logically. By default, there is one VLAN, preconfigured in the switch, VLAN1, that provides access to management functions.

To configure the IP address assigned to the management interface on VLAN 1, enter global configuration mode.
Switch#configure terminal

Next, enter the interface configuration mode for VLAN 1.
Switch(config)#interface vlan 1

Set the IP address, subnet mask, and default gateway for the management interface. The IP address must be valid for the local network where the switch is installed.
Switch(config-if)#ip address
Switch(config)#ip default-gateway

Save the configuration by using the copy running-configuration startup-configuration command.

Connect the Switch to the Network
To connect the switch to a router, use a straight-through cable. LED lights on the switch and router indicate that the connection is successful.

After the switch and router are connected, determine if the two devices are able to exchange messages.

First, check the IP address configuration. Use the show running-configuration command to verify that the IP address of the management interface on the switch VLAN 1 and the IP address of the directly connected router interface are on the same local network.

Then test the connection using the ping command. From the switch, ping the IP address of the directly connected router interface. Repeat the process from the router by pinging the management interface IP address assigned to the switch VLAN 1.

If the ping is not successful, verify the connections and configurations again. Check to ensure that all the cables are correct and that the connections are seated.

After the switch and router are successfully communicating, individual PCs can be connected to the switch using straight-through cables. These cables can be directly connected to the PCs, or can be used as part of the structured cabling leading to wall outlets.

Switch ports can be an entry point to the network by unauthorized users. To prevent this, switches provide a feature called port security. Port security limits the number of valid MAC addresses allowed per port. The port does not forward packets with source MAC addresses that are outside the group of defined addresses.

There are three ways to configure port security.

MAC addresses are manually assigned using the switchport port-security mac-address [mac-address] interface configuration command. Static MAC addresses are stored in the address table and added to the running configuration.

MAC addresses are dynamically learned and stored in the address table. The number of addresses learned can be controlled. By default, the maximum number of MAC addresses learned per port is one. Addresses that are learned are cleared from the table if the port is shutdown or if the switch is restarted.

Similar to dynamic, except that the addresses are also saved to the running configuration.

Port security is disabled by default. If port security is enabled, a violation will result in the port being shutdown. For example, if dynamic port security is enabled and the maximum number of MAC addresses per port is one, the first address learned becomes the secure address. If another workstation attempts to access the port with a different MAC address, a security violation occurs.

There is a security violation when either of these situations occurs:
– The maximum number of secure MAC addresses has been added to the address table, and a device with a MAC address that is not in the address table attempts to access the interface.
– An address learned or configured on one secure interface is seen on another secure interface in the same VLAN.

Before port security can be activated, the port must be set to access mode with the switchport mode access command.

To verify port security settings for the switch or the specified interface, use the show port-security interface interface-id command. The output displays the following:
– Maximum allowed number of secure MAC addresses for each interface
– Number of secure MAC addresses on the interface
– Number of security violations that have occurred
– Violation mode

Additionally, the show port-security address command displays the secure MAC addresses for all ports, and the show port-security command displays the port security settings for the switch.

If static port security or sticky port security is enabled, the show running-config command can be used to view the MAC address associated with a specific port. There are three ways to clear a learned MAC address that is saved in the running configuration:
– Use the clear port-security sticky interface [port-number] access to clear any learned addresses. Next, shutdown the port using the shutdown command. Finally, re-enable the port using the no shutdown command.
– Disable port security using the no switchport port-security interface command. Once disabled, re-enable port security.
– Reboot the switch.

Rebooting the switch will only work if the running configuration is not saved to the startup configuration file. If the running configuration is saved to the startup configuration file, that will eliminate the need for the switch to relearn addresses when the system reboots. However, the learned MAC address will always be associated with a particular port unless the port is cleared using the clear port-security command or disabling port security. If this is done, be sure to re-save the running configuration to the startup configuration file to prevent the switch from reverting to the original associated MAC address upon reboot.

If there are any ports on a switch that are unused, best practice is to disable them. It is simple to disable ports on a switch. Navigate to each unused port and issue the shutdown command. If a port needs to be activated, enter the no shutdown command on that interface.

In addition to enabling port security and shutting down unused ports, other security configurations on a switch include setting passwords on vty ports, enabling login banners, and encrypting passwords with the service password-encryption command. For these configurations, use the same Cisco IOS CLI commands as those used to configure a router.

Cisco Discovery Protocol

Cisco Discovery Protocol (CDP) is an information-gathering tool used on a switch, ISR, or router to share information with other directly connected Cisco devices. By default, CDP begins running when the device boots up. It then sends periodic messages, known as CDP advertisements, onto its directly connected networks.

CDP operates at Layer 2 only and can be used on many different types of local networks, including Ethernet and serial networks. Because it is a Layer 2 protocol, it can be used to determine the status of a directly connected link when no IP address has been configured, or if the IP address is incorrect.

Two Cisco devices that are directly connected on the same local network are referred to as being neighbors. The concept of neighbor devices is important to understand when interpreting the output of CDP commands.

Information gathered by CDP includes:
– Device identifiers – Configured host name
– Address list – Layer 3 address, if configured
– Port identifier – Directly connected port; for example, serial 0/0/0
– Capabilities list – Function or functions provided by the device
-Platform – Hardware platform of the device; for example, Cisco 1841

The output from the show cdp neighbors and show cdp neighbors detail commands displays the information that a Cisco device collects from its directly connected neighbors.

Viewing CDP information does not require logging in to the remote devices. Because CDP collects and displays a lot of information about directly connected neighbors, and no login is required, it is usually disabled in production networks for security purposes. Additionally, CDP consumes bandwidth and can impact network performance.

October 27, 2010

Chapter 4 – Planning the Addressing Structure

Filed under: Uncategorized — krystalchisholm @ 10:07 am

Review of IP Addresses

One of the most important aspects of communications on an internetwork is the IP addressing scheme.
IP addressing is the method used to identify hosts and network devices. As the Internet grew over time and the number of hosts connected to it increased, IP addressing schemes had to adapt to cope with the growth.

While IP addressing schemes have had to adapt, the basic IP address structure for IPv4 remains the same. To send and receive messages on an IP network, every network host must be assigned a unique 32-bit IP address. Because large binary numbers are difficult for people to read and understand, IP addresses are usually displayed in dotted-decimal notation. In dotted-decimal notation, each of the four octets is converted to a decimal number separated by a decimal point. For example, the IP address: 11000000.10101000.00000001.01101010 is represented as in dotted-decimal notation.

IP addresses are hierarchical. For a network, part of the 32-bit number identifies the network (parent), while the rest of the bits identify the host (child). In the early days of the Internet, there were so few organizations needing to connect to the Internet, that networks were assigned by only the first 8 bits (first octet) of the IP address. This left the remaining 24 bits to be used for local host addresses.

The 8-bit network designation made sense at first, because originally people thought that the Internet would be made up of a few very large universities, governments, and military organizations. Using only 8 bits for the network number enabled the creation of 256 separate networks, each containing over 16 million hosts. It soon became apparent that more organizations, and eventually individuals, were connecting to the Internet to do research and to communicate with others. More networks were required, and a way to assign more network numbers had to be created.

To create more possible network designations, the 32-bit address space was organized into five classes. Three of these classes, A, B, and C, provide addresses that can be assigned to individual hosts or networks. The other two classes, D and E, are reserved for multicast and experimental use.

Until this change, routers examined only the first 8-bits of an IP address for the network ID. Class B networks, however, use the first 16 bits to identify the network. Class C networks use the first 24 bits to identify the network. With this addition, routers needed to be programmed to look beyond the first 8 bits to identify class B and C networks.

It was decided to divide the networks in a manner that would make it easy for routers and hosts to determine the correct number of network ID bits. The class of a network is indicated by the values of the first few bits of the IP address, called the high-order bits. If the first bit is 0, the network is a Class A, and the first octet represents the network ID. When the first bit is 1, the router examines the second bit. If that bit is 0, the network is a Class B, and the router uses the first 16 bits for the network ID. If the first three bits are 110, it indicates a Class C address. Class C addresses use the first 24 bits, or three octets, to designate the network. Dividing the original 8-bit network into smaller network classes increased the number of available network designations from 256 to over two million.

In addition to creating separate classes, the Internet Engineering Task Force (IETF) decided to reserve some of the Internet address space for use by private networks. Private networks have no connection to public networks. Private network addresses are not to be routed across the Internet. This allows multiple networks in various locations to use the same private addressing scheme without creating addressing conflicts.

The use of private address space reduced the number of unique registered IP addresses that were assigned to organizations.

A single Class A address,, was reserved for private use. In addition, address space in classes B and C was also set aside for private networks.

Most networks today use a private address structure. Most consumer networking devices, by default, give out private addresses through DHCP. Only the devices that connect directly to the Internet are assigned registered Internet routable addresses.















Subnetting the Network

Networks continued to grow and connect to the Internet throughout the 1980s and into the 1990s, with many organizations adding hundreds, and even thousands, of hosts to their network. An organization with thousands of hosts should have been well served by a Class B network, however, there were some problems.

First, organizations with thousands of hosts rarely had them all in one place. Some organizations wanted to separate individual departments from each other for security or management purposes. Second, a primary type of packet forwarded on a network is the broadcast packet. Broadcast packets are forwarded to all hosts within a single logical network. With thousands of hosts on a single network sending broadcast traffic, and limited bandwidth available, network performance significantly decreased as more hosts were added.

To solve these problems, the organizations leading the development of the Internet chose to partition their networks into mini-networks, or subnetworks, using a process called subnetting. How can a single IP network get split into multiple networks so that each subnet is treated as a separate network?

RFC 917, Internet Subnets, defines the subnet mask as the method routers use to isolate the network portion from an IP address. When a router receives a packet, it uses the destination IP address in the packet and the subnet masks associated with the routes in its routing table to determine the appropriate path on which to forward the packet.

The router reads the subnet mask from left to right, bit by bit. If a bit in the subnet mask is set to 1, it indicates that the value in that position is part of the network ID. A 0 in the subnet mask indicates that the value in that position is part of the host ID.

In the original IP address hierarchy, there are two levels: a network and a host. In a classful addressing scheme, the first three leading bit values are used to determine that an IP address is either a Class A, B, or C. When an address is identified by class, the number of bits that make up the network ID and the number of bits that make up the host ID are known. The default subnet masks for the network classes are:
Class A
Class B
Class C

Subdividing a classful network adds a level to the network hierarchy. Now there are three levels: a network, a subnetwork, and a host. How can the subnet mask be modified to indicate the new hierarchical level?

A single Class A, B, or C network address space can be divided into multiple subnetworks by using bits from the host address space to designate the subnet ID. As an example, an organization using a Class C address space has two offices in different buildings. To make the network easier to manage, the network administrators want each location to have a logically separate network. Taking two bits from the host address increases the subnet mask length from the default 24 bits to 26 bits, or

When bits are borrowed from the host portion of the address to identify the subnet, fewer bits are available for individual hosts. If two bits are used for the subnet ID, only six bits are left in the host portion of the address.

With traditional classful subnetting, the same number of host bits is used to designate the subnet ID for all the resulting subnetworks. This type of subnetting always results in a fixed number of subnets and a fixed number of hosts per subnet. For this reason, this is known as fixed-length subnetting.

The decision about how many host bits to use for the subnet ID is a big planning decision. There are two considerations when planning subnets: the number of hosts on each network, and the number of individual local networks needed. The table for the subnet possibilities for the network shows how the selection of a number of bits for the subnet ID affects both the number of possible subnets and the number of hosts that can be in each subnet.

One thing to keep in mind is that in all IPv4 networks, two host addresses are reserved: the all-0s and the all-1s. An address with all 0s in the host portion of the address is an invalid host address and usually refers to the entire network or subnetwork. An address with all 1s in the host portion is used as the local network broadcast address. When a network is subnetted, each subnet contains an all-0s and an all-1s host address that cannot be used for individual host addresses.

Custom Subnets

When a network is partitioned, the router must use a modified or custom subnet mask to distinguish the subnets from each other.
A default subnet mask and a custom subnet mask differ from each other in that the default subnet masks only change on octet boundaries. For instance, the default subnet mask for a Class A network is Custom subnet masks take bits from the host ID portion of the IP address and add them to the default subnet mask.

To create a custom subnet mask, the first question to answer is how many bits to take from the host ID to add to the subnet mask? The number of bits to borrow to meet a specific number of subnets can be determined by the math equation: 2^n, where n equals the number of bits borrowed.

If three subnets are required, there must be enough subnet bits to allow for three unique subnet addresses.
For example, if starting with a Class C address, such as, there are only eight host bits to borrow from. Each bit can only be a 1 or a 0. To allow for three subnets, at least two of the eight bits must be borrowed. This creates four subnets total:
00 – 1st subnet
01 – 2nd subnet
10 – 3rd subnet
11 – 4th subnet

In the above example, two bits were borrowed, 2^2 = 4 or 2 x 2 = 4, so four subnets were created. If between five and eight subnets were needed, then three bits would be required (2^3 = 8 or 2 x 2 x 2).
The number of bits selected for the subnet ID affects both the number of possible subnets and the number of hosts that can be in each subnet.

With classed subnetting, the number of bits required for the subnet ID depends on two factors: the number of subnets created and the number of hosts per subnet.

In classed, or fixed-length, subnetting, all subnets must be the same size, which means that the maximum number of hosts that each subnet can support is the same for all subnets created. The more bits that are taken for the subnet ID, the fewer bits left for host IDs.

The same base equation, 2^n, with a slight modification, can be used to determine the number of host IDs available based on the number of host bits remaining. Because each subnet has two host addresses that are reserved, the all-0s and all-1s addresses, the equation to determine the number of hosts supported is modified to 2^n – 2.

After it is determined how many bits make up the subnet address, all devices on the network are informed of the subdivision by the subnet mask. With the subnet mask, it is possible to tell which subnet an IP address is in and to design simple classful subnetted IP address schemes.

Subnetting solved a number of problems that existed with the original classed network address spaces. It permitted organizations that owned a class A, B, or C address to subdivide their address space into smaller local subnets to more efficiently assign addresses. However, subnetting is also important in helping to minimize traffic loads and for adding security measures between networks.

An example of a situation that might require subnetting is an ISP customer that has outgrown its initial network installation. In this network, the original small, integrated wireless router is overloaded with traffic from both wired and wireless users. Because of its relatively small size, a Class C address space is used to address the network.

One possible solution to the problem of the overloaded network is to add a second networking device, such as a larger integrated service router (ISR). When adding a device, it is a good practice to place the wired and wireless users on separate local subnetworks to increase security. The original wireless router can still be used to provide the wireless users with connectivity and security on one network. Hubs or switches connecting the wired users can then be directly connected to the new ISR using a different network. The ISR and the wireless router can then be directly connected with a third network.

This new network configuration requires that the existing Class C network be divided into at least three subnetworks. Using classful subnetting, at least two bits must be taken from the host portion of the address to meet the customer requirements. This subnetting scheme results in the creation of four individual networks, each with 62 available host addresses (64 possible addresses, minus the all-0s and all-1s addresses).

VLSM and Classless Inter-Domain Routing (CIDR)

The original classful subnetting design required that all subnets of a single classed network be the same size. This was because routers did not include subnet mask information in their routing updates. A router programmed with one subnet address and mask on an interface automatically applied that same mask to the other network subnets in its routing table. This limitation required planning for fixed-length subnet masks in the IP addressing scheme.

However, fixed-length subnet masks can waste a significant number of IP addresses. For example, an organization with one site has approximately 8,000 hosts and three other locations with 1,000, 400, and 100 hosts, respectively. With a fixed-length subnet mask, each subnet would have to support at least 8,000 hosts, even the one assigned to the location needing only 100 addresses.

Variable length subnet masking (VLSM) helps to solve this issue. VLSM addressing allows an address space to be divided into networks of various sizes. This is done by subnetting subnets. To accomplish this, routers today must receive routing information that includes the IP address of the network, and the subnet mask information which indicates the number of bits that make up the network portion of the IP address. VLSM saves thousands of IP addresses that would be wasted with traditional classful subnetting.

In addition to VLSM, Classless Inter-Domain Routing (CIDR) was proposed in RFC 1519 and accepted. CIDR ignores network classes based on the value of the high-order bits. CIDR identifies networks based solely on the number of bits in the network prefix, which corresponds to the number of 1s in the subnet mask. An example of an IP address written using CIDR notation is, where the /16 represents the number of bits in the network prefix.

CIDR protocols freed routers from using only the high-order bits to determine the network prefix. Removing that restriction eliminated the need to allocate registered IP addresses by address class.

Before CIDR, an ISP requiring 3,000 host addresses could request either a full Class B address space or multiple Class C network addresses to meet its requirements. With a Class B address space, the ISP would waste thousands of registered addresses. If it requested multiple Class C addresses, it could be difficult to design the ISP network so that no single section required more than 254 host addresses. Routing tables containing many Class C addresses can also get large and difficult to manage.

By ignoring the traditional address classes, CIDR enables the ISP to request a block of addresses based on the number of host addresses it requires. Supernets, created by combining a group of Class C addresses into one large block, enable addresses to be assigned more efficiently. An example of a supernet is Using the first 19 bits of the IP address for the network prefix enables this supernet to contain 8,190 possible host addresses. An ISP can use a supernet as one large network or divide it into as many smaller networks as needed to meet its requirements.

In this example of a supernet, the private Class C address of is used. In reality, most networks that use private addressing use either the Class A or B reserved addresses and subnetting. Although classed addressing and fixed-length subnet masking are becoming less common, it is important to understand how these addressing methods work. Many devices still use the default subnet mask if no custom subnet mask is specified.

Communicating Between Subnets

When a network is split into subnets, each subnet is actually a completely separate network. Therefore, for a device in one subnet to communicate with a device in another subnet, a router is required because routers connect networks.

To determine how many hosts are needed in each subnet, it is necessary to include the router interface, or gateway interface, and the individual host devices. Each router interface must have an IP address in the same subnet as the host network attached to it.

In some instances, it may be necessary to connect two routers, such as when connecting the Linksys device and the 1841 ISR. This configuration must ensure that interfaces on routers that connect to each other are assigned IP addresses in the same network or subnet. Here the common link shows the two routers connected on the subnet with host IP addresses of and

Basic Network Address Transitions (NAT)

Routers are required to route between subnets on an internal network, regardless of whether the IP address range is public or private. However, if the address range is private, private networks cannot be routed across the public Internet. Therefore, how do host devices using a private addressing scheme communicate across the Internet? Network Address Translation (NAT) must be enabled on the device connecting the private network to the ISP network.

NAT allows a large group of private users to access the Internet by sharing one or more public IP addresses. Address translation is similar to how a telephone system works in a company. As a company adds employees, at some point, they no longer run a public phone line directly to each employee desk. Instead, they use a system that allows the company to assign each employee an extension number. The company can do this because not all employees use the phone at the same time. Using private extension numbers enables the company to purchase a smaller number of external phone lines from the phone company.

NAT works similarly to a company phone system. Saving registered IP addresses is one of the main reasons that NAT was developed. NAT can also provide security to PCs, servers, and networking devices by withholding their actual IP host addresses from direct Internet access.

The main advantages of NAT are that IP addresses can be re-used and many hosts on a single LAN can share globally unique IP addresses. NAT operates transparently and helps shield users of a private network against access from the public domain.

In addition, NAT hides private IP addresses from public networks. The advantage to this is that NAT operates much like an access control list, not allowing outside users to access internal devices. The disadvantage is that additional configurations are required to allow access from legitimate, external users.

Another disadvantage is that NAT has an impact on some applications that have IP addresses in their message payload, because these IP addresses must also be translated. This translation increases load on the router and hinders network performance.

When configuring NAT on a router, there are a few terms that help explain how the router accomplishes NAT:
Inside local network – Refers to any network connected to a router interface that is part of the privately addressed LAN. Hosts on inside networks have their IP addresses translated before they are transmitted to outside destinations.
Outside global network – Any network attached to the router that is external to the LAN and does not recognize the private addresses assigned to hosts on the LAN.
Inside local address – Private IP address configured on a host on an inside network. The address must be translated before it can travel outside the local network addressing structure.
Inside global address – IP address of an inside host as it appears to the outside network. This is the translated IP address.
Outside local address – Destination address of the packet while it is on the local network. Usually, this address is the same as the outside global address.
Outside global address – Public IP address of an external host. The address is allocated from a globally routable address or network space.

Static and Dynamic NAT

Addresses can be assigned dynamically. Dynamic NAT allows hosts on a private network that have private IP addresses to access a public network, such as the Internet. Dynamic NAT occurs when a router assigns an outside global address from a pre-defined address, or pool of addresses, to an inside private network device.

As long as the session is open, the router watches for the inside global address and sends acknowledgments to the initiating inside device. When the session ends, the router simply returns the inside global address to the pool.

One of the advantages of using NAT is that individual hosts are not directly accessible from the public Internet. But what if one or more of the hosts within a network are running services that need to be accessed from Internet connected devices and devices on the local private LAN?

One way to provide access to a local host from the Internet is to assign that device a static address translation. Static translations ensure that an individual host private IP address is always translated to the same registered global IP address. It ensures that no other local host is translated to the same registered address.

Static NAT allows hosts on the public network to access selected hosts on a private network. If a device on the inside network needs to be accessible from the outside, use static NAT.

Both static and dynamic NAT can be configured at the same time, if necessary.

Port-based Network Address Translation (PAT)

When an organization has a very small registered IP address pool, or perhaps even just a single IP address, it can still enable multiple users to simultaneously access the public network with a mechanism called NAT overload, or Port Address Translation (PAT). PAT translates multiple local addresses to a single global IP address.

When a source host sends a message to a destination host, it uses an IP address and port number combination to keep track of each individual conversation with the destination host. In PAT, the gateway translates the local source address and port combination in the packet to a single global IP address and a unique port number above 1024. Although each host is translated into the same global IP address, the port number associated with the conversation is unique.

Responding traffic is addressed to the translated IP address and port number used by the host. A table in the router contains a list of the internal IP address and port number combinations that are translated to the external address. Responding traffic is directed to the appropriate internal address and port number. Because there are over 64,000 ports available, a router is unlikely to run out of addresses, which could happen with dynamic NAT.

Because each translation is specific to the local address and local port, each connection, which generates a new source port, requires a separate translation. For example, requires a separate translation from

The translation is only in place for the duration of the connection, so a given user does not keep the same global IP address and port number combination after the conversation ends.

Users on the outside network cannot reliably initiate a connection to a host on a network that uses PAT. Not only is it impossible to predict the local or global port number of the host, but a gateway does not even create a translation unless a host on the inside network initiates the communication.

IP NAT Issues

People access the Internet from private networks without ever realizing that the router is using NAT. However, an important issue with NAT is the additional workload necessary to support IP address and port translations.

Some applications increase the workload of the router, because they embed an IP address as part of the encapsulated data. The router must replace the source IP addresses and port combinations that are contained within the data, and the source addresses in the IP header.

With all this activity taking place within a router, NAT implementation requires good network design, careful selection of equipment and accurate configuration.

NAT has become so commonplace in integrated networking devices used in homes and small businesses, that for some people, configuring it is a matter of selecting a check box. As businesses grow and require more sophisticated gateway and routing solutions, device configurations for NAT become more complex.

Subnetting networks, private IP addressing, and the use of NAT were developed to provide a temporary solution to the problem of IP address depletion. These methods, though useful, do not create more IP addresses. As a response to address depletion, IPv6 was proposed in 1998 with RFC 2460.

Although its primary purpose was to solve IPv4 IP address depletion, there were other good reasons for its development. Since IPv4 was first standardized, the Internet has grown significantly. This growth has uncovered advantages and disadvantages of IPv4, and the possibility for upgrades to include new capabilities.

A general list of improvements that IPv6 proposes are:
– More address space
– Better address space management
– Easier TCP/IP administration
– Modernized routing capabilities
– Improved support for multicasting, security, and mobility

The development of IPv6 is designed to address as many of these requests and problems as possible.

With IPv6, IP addresses are 128 bits with a potential address space of 2^128. In decimal notation, that is approximately a 3 followed by 38 zeroes. If IPv4 address space was represented by a small marble, then IPv6 address space is represented by a volume almost equivalent to the planet Saturn.

Working with 128-bit numbers is difficult, so the IPv6 address notation represents the 128 bits as 32 hexadecimal digits, which are further subdivided into eight groups of four hexadecimal digits, using colons as delimiters. The IPv6 address has a three-part hierarchy. The global prefix is the first three blocks of the address and is assigned to an organization by an Internet names registry. The subnet and the interface ID are controlled by the network administrator.

Network administrators will have some time to adjust to this new IPv6 structure. Before the widespread adoption of IPv6 occurs, network administrators still need a way to more efficiently use private address spaces.

October 13, 2010

Chapter 3 – Planning a Network Upgrade

Filed under: Uncategorized — krystalchisholm @ 9:46 am

Site Survey

When you have a smaller network in a business, it usually works fine. But as the business expands, the network will need to as well. And if this is done without knowledge of the way networks function, then this can end with a large network amidst a jungle of wires and failed connections.

Most businesses will consult outside help for the network upgrade. But before any real planning can happen, there needs to be a site survey.

A site survey provides the network designer important information and creates a proper starting point for the project. It shows what is already on site, and gives a good indication as to what is needed.

Important pieces of information that can be gathered during a site survey include:
– Number of users and types of equipment
– Projected growth
– Current Internet connectivity
– Application requirements
– Existing network infrastructure and physical layout
– New services required
– Security and privacy considerations
– Reliability and uptime expectations
– Budget constraints

It is a good idea to obtain a floor plan, if possible. If a floor plan is not available, the technician can draw a diagram indicating the size and location of all rooms. An inventory of existing network hardware and software is also useful to provide a baseline of requirements for the upgrade.

A sales representative may also accompany the technician to the site to interview the customer. The sales representative may ask a series of questions to gather information about the network upgrade needs of the business.

The technician should be prepared for anything when doing the site survey. Networks do not always meet local codes of practice in terms of electrical, building, or safety regulations, nor adhere to any standards.

Sometimes networks grow haphazardly over time and end up being a mixture of technologies and protocols. The technician should be careful not to offend the customer by expressing an opinion about the quality of the existing installed network.

When visiting the customer premises, the technician should do a thorough overview of the network and computer setup. There may be some obvious issues such as unlabeled cables, poor physical security for network devices, lack of emergency power, or lack of an uninterruptible power supply (UPS) for critical devices. These conditions are noted in the site survey report, in addition to the other requirements gathered from the survey and the customer interview.

When the site survey is completed, it is important that the technician review the results with the customer to ensure that nothing is missed and that there are no errors. If everything is accurate, the site survey provides an excellent basis for the new network design.

Physical and Logical Topographies

Both the physical and logical topology of the network must be documented. A physical topology is the actual physical location of cables, computers, and other peripherals. A logical topology documents the path that data takes through the network and where network functions, like routing, occur. A technician gathers this information during the site survey to create the physical and logical topology map.

In a wired network, the physical topology map consists of the wiring closet and the wiring to the individual end-user stations. In a wireless network, the physical topology consists of the wiring closet and an access point. Because there are no wires, the physical topology contains the wireless signal coverage area.
The logical topology is generally the same for a wired and wireless network. It includes the naming and Layer 3 addressing of end stations, router gateways, and other network devices, regardless of the physical location. It indicates the location of routing, network address translation, and firewall filtering.

To develop a logical topology requires understanding the relationship between the devices and the network, regardless of the physical cabling layout. There are several topological arrangements possible. Examples include star, extended star, partial mesh, and full mesh topologies.

Star Topologies
With a star topology, each device is connected via a single connection to a central point. The central point is typically a switch or a wireless access point. The advantage of a star topology is that if a single connecting device fails, only that device is affected. However, if the central device, such as the switch, fails, then all connecting devices lose connectivity.
An extended star is created when the central device in one star is connected to a central device of another star, such as when multiple switches are interconnected, or daisy-chained together.
Mesh Topologies
Most Core Layers in a network are wired in either a full mesh or a partial mesh topology. In a full mesh topology, every device has a connection to every other device. While full mesh topologies provide the benefit of a fully redundant network, they can be difficult to wire and manage and are more costly.
For larger installations, a modified partial mesh topology is used. In a partial mesh topology, each device is connected to at least two other devices. This arrangement creates sufficient redundancy, without the complexity of a full mesh.

Implementing redundant links through partial or full mesh topologies ensures that network devices can find alternate paths to send data in the event of a failure.

Network Requirements Documentation

Along with creating the topology maps for the existing network, it is necessary to obtain additional information about the hosts and networking devices that are currently installed. This information is recorded on a brief inventory sheet. The technician also documents any growth that the company anticipates in the near future.
This information helps the network designer determine what new equipment is required, and the best way to structure the network to support the anticipated growth.

The inventory sheet of the installed devices includes:
– Device name
– Date of purchase
– Warranty information
– Location
– Brand and model
– Operating system
– Logical addressing information
– Gateway
– Method of connectivity
– Virus Checker
– Security information

Network Upgrade Planning Phase

A network upgrade requires extensive planning. Just like any project, a need is identified and then a plan outlines the process from beginning to end. A good project plan helps identify any strengths, weaknesses, opportunities, or threats (SWOT). The plan clearly defines the tasks, and the order in which the tasks are to be completed.
A network that is a patchwork of devices strung together, using a mixture of technologies and protocols, is usually an indicator of poor initial planning. These types of networks are susceptible to downtime, and are difficult to maintain and troubleshoot.

Planning a network upgrade begins after the site survey and the resulting report are completed. There are five distinct phases.
Phase 1: Requirements Gathering
After all of the information has been gathered from the customer and the site visit, it is analyzed to determine the network requirements. This analysis is done by the design team at the ISP, which creates an Analysis Report.
Phase 2: Selection and Design
Devices and cabling are selected based on the requirements outlined in the Analysis Report. Multiple design options are created and regularly shared with other members on the project. This phase allows team members to view the network from a documentation perspective and evaluate trade-offs in performance and cost. It is during this step that any weaknesses of the design can be identified and addressed.
Also during this phase, prototypes are created and tested. A prototype is a good indicator of how the new network will operate.
When the design is approved by the customer, implementation of the new network can begin.
Phase 3: Implementation
If the first two steps are done correctly, the implementation phase is more likely to be performed without incident. If there are tasks that have been overlooked in the earlier phases, they must be corrected during implementation. Creating an implementation schedule that allows time for unexpected events, keeps disruption for the customer to a minimum. Staying in constant communication with the customer during the installation is critical to the success of the project.
Phase 4: Operation
The network is brought into service in what is called a production environment. Prior to this step, the network is considered to be in a testing or implementation phase.
Phase 5: Review and Evaluation
After the network is in operation, the design and implementation must be reviewed and evaluated. For this process, the following steps are recommended:
– Step 1: Compare the user experience with the goals in the documentation, and evaluate if the design is right for the job.
– Step 2: Compare the projected designs and costs with the actual deployment. This evaluation ensures that future projects will benefit from the lessons learned on this project.
– Step 3: Monitor the operation and record changes. It is important that the system is always fully documented and accountable.
Careful planning at each phase ensures that the project goes smoothly and that the installation is successful. On-site technicians are often included in the planning, because they participate in all phases of the upgrade.

Physical Environment

One of the first things that the network designer does to select the equipment and design of the new network is to examine the existing network facilities and cabling. The facilities include the physical environment, the telecommunication room, and the existing network wiring. A telecommunications room, or wiring closet, in a small, single-floor network is usually referred to as the Main Distribution Facility (MDF).
The MDF typically contains many of the network devices, including switches or hubs, routers, and access points. It is where all of the network cable concentrates to a single point. Many times, the MDF also contains the Point of Presence (POP) of the ISP, where the network makes the connection to the Internet through a telecommunications service provider.

If additional wiring closets are required, they are referred to as Intermediate Distribution Facilities (IDFs). IDFs are typically smaller than the MDF, and connect to the MDF.
Many small businesses do not have a telecommunications room or closet. Network equipment may be located on a desk or other furniture, and wires could be just lying on the floor. Network equipment must always be secure. As a network grows, a telecommunications room is critical to the security and reliability of the network.

Cabling Considerations

When the existing cabling is not up to specification for the new equipment, new cabling must be planned for and installed. The condition of the existing cabling can quickly be determined by the physical inspection of the network during the site visit. When planning the installation of network cabling, there are four physical areas to consider:
– User work areas
– Telecommunications room
– Backbone area
– Distribution area

There are many different types of cable found in the networking environment, and some are more common than others:
Shielded twisted pair (STP) – Usually Category 5, 5e, or 6 cable that has a foil shielding to protect from outside electromagnetic interference (EMI). In an Ethernet environment, the distance limitation is approximately 328 feet (100 meters).
Unshielded twisted pair (UTP) – Usually Category 5, 5e, or 6 cable that does not provide extra shielding from EMI, but it is inexpensive. Cable runs should avoid electrically noisy areas. In an Ethernet environment, the distance limitation is approximately 328 feet (100 meters).
Fiber-optic cable – A medium that is not susceptible to EMI, and can transmit data faster and farther than copper. Depending on the type of fiber optics, distance limitations can be several miles (kilometers). Fiber-optic can be used for backbone cabling and high-speed connections.

In addition to these three commonly-used cabling types, coaxial is also used in networking. Coaxial is not typically used in LANs, but it is widely used in cable modem provider networks. Coaxial has a solid copper core with several protective layers including polyvinyl chloride (PVC), braided wire shielding, and a plastic covering. Distance is several miles (kilometers). Limitations depend on the purpose of the connection.

There are several organizations in the world that provide LAN cabling specifications.
The Telecommunications Industry Association (TIA) and the Electronic Industries Alliance (EIA) worked together to provide the TIA/EIA cable specifications for LANs. Two of the most common TIA/EIA cable specifications include the 568-A and 568-B standards. Both of these standards typically use the same Cat 5 or Cat 6 cable, but with a different termination color code.

There are three different types of twisted pair cables that are used in networks:
Straight-through – Connects dissimilar devices, such as a switch and a computer, or a switch and a router.
Crossover – Connects similar devices, such as two switches or two computers.
Console (or Rollover) – Connects a computer to the console port of a router or switch to do initial configuration.

Another cable type that is common in networks is a serial cable. A serial cable is typically used to connect the router to an Internet connection. This Internet connection may be to the phone company, the cable company, or a private ISP.

Structured Cable

When designing a structured cable project, the first step is to obtain an accurate floor plan. The floor plan allows the technician to identify possible wiring closet locations, cable runs, and which electrical areas to avoid.

After the technician has identified and confirmed the locations of network devices, it is time to draw the network on the floor plan. Some of the more important items to document include the following:
Patch cable – Short cable from the computer to the wall plate in the user work area
Horizontal cable – Cable from the wall plate to the IDF in the distribution area
Vertical cable – Cable from the IDF to the MDF in the backbone area of the business
Backbone cable – Network part that handles the major traffic
Location of wiring closet – Area to concentrate the end-user cables to the hub or switch
Cable management system – Trays and straps used to guide and protect cable runs
Cable labeling system – Labeling system or scheme to identify cables
Electrical considerations – Outlets and other items to support the electrical requirements of the network equipment

Purchasing Equipment

As the ISP team plans the network upgrade, issues related to purchasing new equipment and the maintenance of new and existing equipment must be addressed. There are generally two options for obtaining new equipment:
Managed service – The equipment is obtained from the ISP through a lease or some other agreement, and the ISP is responsible for updating and maintaining the equipment.
In-house – The customer purchases the equipment, and the customer is responsible for the updates, warranties, and maintenance of the equipment.
When acquiring equipment, cost is always a major factor. A good cost analysis of the various options provides a sound basis for the final decision.
If a managed service is chosen, there are lease costs and possibly other service costs as outlined in the Service Level Agreement (SLA).
If the equipment is purchased outright, the customer should be aware of the price of the equipment, warranty coverage, compatibility with existing equipment, and update and maintenance issues. All of these must be analyzed to determine the cost-effectiveness of the purchase.

Selecting Network Devices

After analyzing requirements, the design staff recommends the appropriate network devices to connect and support the new network functionality.

Modern networks use a variety of devices for connectivity. Each device has certain capabilities to control the flow of data across a network. A general rule is that the higher the device is in the OSI model, the more intelligent it is. What this means is that a higher level device can better analyze the data traffic and forward it based on information not available at lower layers. As an example, a Layer 1 hub can forward data only out of all ports, while a Layer 2 switch can filter the data and send it only out of the port that is connected to the destination based on the MAC address.

As switches and routers evolve, the distinction between them may seem blurred. One simple distinction remains: LAN switches provide connectivity within the local-area networks of the organization, while routers interconnect local networks and are needed in a wide-area network environment.

In addition to switches and routers, there are other connectivity options available for LANs. Wireless access points allow computers and other devices, such as handheld IP phones, to wirelessly connect to the network or share broadband connectivity. Firewalls guard against network threats and provide security and network control and containment.

Integrated Service Routers (ISRs) are network devices that combine the functionality of switches, routers, access points, and firewalls into the same device.

Selecting LAN Devices

Although both a hub and a switch can provide connectivity at the Access Layer of a network, switches should be chosen for connecting devices to a LAN. Switches are more expensive than hubs, but the enhanced performance makes switches more cost-effective. A hub is generally chosen as a networking device only within a very small LAN, a LAN that requires little throughput requirements, or when finances are limited.

When selecting a switch for a particular LAN, there are a number of factors to consider. These factors include, but are not limited to:
– Speed and the types of ports and interfaces involved
– Expandability
– Manageability
– Cost

Speed and Types of Ports and Interfaces
Choosing Layer 2 devices that can accommodate increased speeds allows the network to evolve without replacing the central devices.
When selecting a switch, choosing the appropriate number and type of ports is critical.
Network designers should consider carefully how many twisted pair (TP) and fiber-optic ports are needed. It is also important to estimate how many more ports will be required to support network expansion.

Networking devices come in both fixed and modular physical configurations. Fixed configurations have a specific type and number of ports or interfaces. Modular devices have expansion slots that provide the flexibility to add new modules as requirements evolve. Most modular devices come with a minimum number of fixed ports and expansion slots.
A typical use of an expansion slot is to add fiber-optic modules to a device originally configured with a number of fixed TP ports. Modular switches can be a cost-effective approach to scaling LANs.

A basic, inexpensive switch is not configurable. A managed switch that uses a Cisco IOS feature set allows control over individual ports or over the switch as a whole. Controls include the ability to change the settings for a device, add port security, and monitor performance.
For example, with a managed switch, ports can be turned on or off. In addition, administrators can control which computers or devices are allowed to connect to a port.

The cost of a switch is determined by its capacity and features. The switch capacity includes the number and types of ports available and the overall throughput. Other factors that affect the cost are network management capabilities, embedded security technologies, and advanced switching technologies.
Using a simple cost-per-port calculation, it may initially appear that the best option is to deploy one large switch at a central location. However, this apparent cost savings may be offset by the expense of the longer cable lengths required to connect every device on the LAN to one switch. This option should be compared with the cost of deploying a number of smaller switches connected by a few long cables to a central switch.
Deploying a number of smaller devices, instead of a single large device, also has the benefit of reducing the size of the failure domain. A failure domain is the area of the network affected when a piece of networking equipment malfunctions or fails.

After the LAN switches are selected, determine which router is appropriate for the customer.

Selecting Internetworking Devices

A router is a Layer 3 device. It performs all tasks of devices in lower layers and selects the best route to the destination based on Layer 3 information. Routers are the primary devices used to interconnect networks. Each port on a router connects to a different network and routes packets between the networks. Routers have the ability to break up broadcast domains and collision domains.

When selecting a router, it is necessary to match the characteristics of the router to the requirements of the network. Factors for choosing a router include:
– Type of connectivity required
– Features available
– Cost

Routers interconnect networks that use different technologies. They can have both LAN and WAN interfaces.
The LAN interfaces of the router connect to the LAN media. The media is typically UTP cabling, but modules can be added for using fiber optics. Depending on the series or model of router, there can be multiple interface types for connecting LAN and WAN cabling.

It is necessary to match the characteristics of the router to the requirements of the network. After analysis, the business management may determine that it needs a router with specific features. In addition to basic routing, features include:
– Security
– Quality of Service (QoS)
– Voice over IP (VoIP)
– Network Address Translation (NAT)
– Dynamic Host Configuration Protocol (DHCP)
– Virtual Private Network (VPN)

Budget is an important consideration when selecting internetwork devices. Routers can be expensive, and additional modules, such as fiber optic modules, can increase the cost.
An Integrated Service Router (ISR) is a relatively new technology that combines multiple services into one device. Before the introduction of the ISR, multiple devices were required to meet the needs of data, wired, wireless, voice, video, firewall, and VPN technologies. The ISR was designed with multiple services to accommodate the demands of small- to medium-sized businesses and branch offices of large organizations. With an ISR, an organization can quickly and easily enable end-to-end protection for users, applications, network endpoints, and wireless LANs. In addition, the cost of an ISR can be less than if the individual devices were purchased separately.

Network Equipment Upgrades

Many small networks were initially built using a low-end integrated router to connect wireless and wired users. These routers are designed to support small networks, usually consisting of a few wired hosts and possibly four or five wireless devices. When a small business outgrows the capabilities of their existing network devices, it is necessary to upgrade to more robust devices. Within this course, examples of these devices are the Cisco 1841 ISR and the Cisco 2960 Switch.
The Cisco 1841 is designed to be a branch office or medium-sized business router. As an entry-level multiservice router, it offers a number of different connectivity options. It is modular in design and can deliver multiple security services.

Some of the features of the Catalyst 2960 switches are:
– Entry-level, enterprise-class, fixed-configuration switching that is optimized for Access Layer deployments
– Fast Ethernet and Gigabit Ethernet to desktop configurations
– Ideal for entry-level enterprise, mid-market, and branch-office environments
– Compact size for deployments outside of the wiring closet
These switches can provide the high speeds and high-density switching capabilities that the smaller ISRs with integrated switching cannot. They are a good option when upgrading networks built with either hubs or small ISR devices.
The Cisco Catalyst 2960 Series Intelligent Ethernet Switches are a family of fixed-configuration, standalone devices that provide Fast Ethernet and Gigabit Ethernet connectivity to the desktop.

Design Considerations

Purchasing network devices and installing cables are only the beginning of the network upgrade process. Networks must also be reliable and available. Reliability can be achieved by adding redundant components to the network, such as two routers instead of one. In this instance, alternate data paths are created, so if one router is experiencing problems, the data can take an alternate route to arrive at the destination.

An increase in reliability leads to improved availability. For example, telephone systems require five-9s of availability. This means that the telephone system must be available 99.999% of the time. Telephone systems cannot be down, or unavailable, for more than .001% of the time.

Fault tolerance systems are typically used to improve network reliability. Fault tolerance systems include devices such as a UPS, multiple AC power supplies, hot-swappable devices, multiple interface cards, and backup systems. When one device fails, the redundant or backup system takes over to ensure minimal loss of reliability. Fault tolerance can also include backup communication links.

IP Addressing Plan
Planning for a network installation must include planning the logical addressing. Changing the Layer 3 IP addressing is a major issue when upgrading a network. If the structure of the network is going to be changed in the upgrade, the IP address scheme and network information may need to be altered.

The plan should include every device that requires an IP address, and account for future growth. The hosts and network devices that require an IP address include:
– User computers
– Administrator computers
– Servers
– Other end devices such as printers, IP phones, and IP cameras
– Router LAN interfaces
-Router WAN (serial) interfaces

There are other devices that may need an IP address to access and manage them. These include:
– Standalone switches
– Wireless Access Points

For example, if a new router is introduced to the network, each interface on that router can be used to create additional networks, or subnets. These new subnets need to have the proper IP address and subnet mask calculated. Sometimes, this means having to assign a totally new addressing scheme to the network.

After all of the planning and design phases are complete, the upgrade proceeds to the implementation phase, in which the actual network installation begins.


October 8, 2010

Chapter 2 – Help Desk

Filed under: Uncategorized — krystalchisholm @ 10:14 am

ISP Help Desk Organisation

ISPs provide the Internet connection for businesses, and they provide their customers support for problems that occur with Internet connectivity. This support usually includes assistance with customer equipment problems. ISP support is typically provided through the ISP help desk. Whether the problem is connecting to the Internet or getting email, the ISP help desk is usually the first place a user or business turns to for help.

At an ISP, there are usually three levels of customer support:
– Level 1 is for immediate support handled by junior-level help desk technicians.
– Level 2 handles calls that are escalated to more experienced telephone support.
– Level 3 is for calls that cannot be resolved by phone support and require a visit by an on-site technician.

In addition to ISPs, many other types of medium to large businesses employ help desk or customer support teams. The titles assigned to the technicians may vary from those described here, although the three-level hierarchy is the most common structure. Depending on the size of the organization, the help desk can consist of one person that performs all three levels of support, or it can be a comprehensive call center with elaborate call routing facilities and escalation rules. Some ISPs and businesses contract out their help desk functions to a third-party call center company, which provides the services of Level 1 and Level 2 technicians.

Roles of ISP Technicians

When a user initially contacts the help desk to resolve an issue, the call or message is usually directed to a Level 1 support technician. Level 1 support is usually an entry-level position that provides junior technicians with valuable experience. Many customer issues are resolved by the Level 1 support technician.

Issues that cannot be resolved are sent to Level 2 support, which typically has fewer agents available. The duties and responsibilities of the Level 2 technician are similar to that of the Level 1 technician, but they are at a higher skill level. These agents are expected to solve problems that are more challenging and require more knowledge.

Many larger service providers have expanded their businesses to include managed services or on-site support of a customer network. Organizations that provide managed services are sometimes referred to as Managed Service Providers (MSP). Managed services can be provided by ISPs, telecommunications service providers, or other types of computer and network support organizations. When an ISP is providing managed services, it often requires technicians to visit customer sites for the purpose of installation and support. This type of service represents Level 3 support.

Level 3 support is usually in accordance with a Service Level Agreement (SLA). An SLA resembles an insurance policy, because it provides coverage or service if there is a computer or network problem.

Interacting with the Customers

Help desk technicians may be required to provide phone support, email support, web-based support, online chat support, and possibly on-site support. They are often the first point of contact for frustrated and anxious customers. Until a problem is solved, help desk technicians may continue to get calls and correspondence asking for status updates and time estimates to resolve an issue.

The help desk technician must be able to stay focused in an environment with frequent interruptions and perform multiple tasks efficiently and accurately. It can be difficult to consistently maintain a positive attitude and provide a high level of service. The help desk technician has to have excellent interpersonal skills and effective communication skills, both oral and written. The technician must be able to work independently and as part of a team.
It is important for the help desk technician to be able to handle customer issues with speed, efficiency, and professionalism. Help desk technicians should conduct themselves in accordance with the company’s customer service philosophy. A customer service philosophy is an organization-wide ethic shared by everyone from top management to operational staff.

Basic incident management procedures must be followed every time a help desk technician receives a call and begins troubleshooting issues. Incident management includes opening a trouble ticket and following a problem-solving strategy. Problem-solving techniques include using troubleshooting flowcharts, addressing questions in a template format, and maintaining proper ticket escalation procedures.
A help desk script is used by the help desk technician to gather information and cover the important facts about customer incident.

In addition to technical ability, help desk technicians must be able to greet customers pleasantly and be professional and courteous throughout the call.
Customer service and interpersonal skills are especially important when handling difficult clients and incidents. The help desk technician must know how to relieve customer stress and respond to abusive customers.
Opening trouble tickets and logging information on the tickets are critical to help desk operation. When there are many calls relating to a single problem or symptom, it is helpful to have information on how the problem was resolved in the past. It is also important to relay to the customer what is being done to solve the problem. Good information on open trouble tickets helps communicate accurate status, both to the customer and other ISP personnel.

While many issues can be handled remotely, some problems require an on-site visit to the customer premises to install and troubleshoot equipment. When a technician goes on-site, it is important to represent their organization in a professional manner. A professional knows how to make the customer feel at ease and confident in the technician’s skills.

On the first visit to a customer location, it is important for the technician to make a good impression. Personal grooming and the way the technician is dressed are the first things the customer notices. If the technician makes a bad first impression, it may be difficult to change that impression and gain the confidence of the customer. Many employers provide a uniform or have a dress code for their on-site technicians.
The language and attitude of the technician also reflect on the organization that the technician represents. A customer may be anxious or concerned about how the new equipment will operate. When speaking with a customer, the technician should be polite and respectful, and answer all customer questions. If the technician does not know an answer to a customer question or if additional information is required, the technician should write down the customer inquiry and follow up on it as soon as possible.

Using the OSI Model

When a network connectivity problem is reported to the help desk, many methods are available to diagnose the problem.

One common method is to troubleshoot the problem using a layered approach. A layered approach requires that the network technician be familiar with the various functions that occur as messages are created, delivered, and interpreted by the network devices and hosts on the network.

Moving data across a network is best visualized using the seven layers of the Open Systems Interconnection model, commonly referred to as the OSI model. The OSI model breaks network communications down into multiple processes. Each process is a small part of the larger task.

The seven layers of the OSI model are divided into two parts: upper layers and lower layers.

The term upper layer is sometimes used to refer to any layer above the Transport layer of the OSI model. The upper layers deal with application functionality and are generally implemented only in software. The highest layer, the Application layer, is closest to the end user.

The term lower layer is sometimes used to refer to any layer below the Session layer. The combined functionality of the lower layers handles data transport. The Physical layer and the Data Link layer are implemented in both hardware and software. The Physical layer is closest to the physical network medium, or network cabling. The Physical layer actually places information on the medium.

End stations, like clients and servers, usually work with all seven layers. Networking devices are only concerned with the lower layers. Hubs work on Layer 1, switches on Layers 1 and 2, routers on Layers 1, 2 and 3, and firewalls on Layers 1, 2, 3, and 4.

OSI Model Protocols and Technologies

The OSI model divides the task of sending and receiving email into smaller, distinct steps that correspond with the seven layers.
Step 1: Upper layers create the data.

When a user sends an email message, the alphanumeric characters within the message are converted to data that can travel across the network. Layers 7, 6, and 5 are responsible for ensuring that the message is placed in a format that can be understood by the application running on the destination host. This process is called encoding. The upper layers then send the encoded messages to the lower layers for transport across the network. Transporting the email to the correct server relies on the configuration information provided by the user. Problems that occur at the application layer are often related to errors in the configuration of the user software programs.

Step 2: Layer 4 packages the data for end-to-end transport.
The data that comprises the email message is packaged for network transport at Layer 4. Layer 4 breaks the message down into smaller segments. A header is placed on each segment indicating the TCP or UDP port number that corresponds to the correct application layer application. Functions in the transport layer indicate the type of delivery service. Email utilizes TCP segments, therefore packet delivery is acknowledged by the destination. Layer 4 functions are implemented in software that runs on the source and destination hosts. However, because firewalls often use the TCP and UDP port numbers to filter traffic, problems that occur at Layer 4 can be caused by improperly configured firewall filter lists.

Step 3: Layer 3 adds the network IP address information.
The email data received from the transport layer is put into a packet that contains a header with the source and destination network IP addresses. Routers use the destination address to direct the packets across the network along the appropriate path. Incorrectly configured IP address information on the source or destination system can cause Layer 3 problems to occur. Because routers also use IP address information, router configuration errors can cause problems at this layer.

Step 4: Layer 2 adds the data link layer header and trailer.
Each network device in the path from the source to the destination, including the sending host, encapsulates the packet into a frame. The frame contains the physical address of the next directly-connected network device on the link. Each device in the chosen network path requires framing so that it can connect to the next device. Switches and network interface cards (NICs) use the information in the frame to deliver the message to the correct destination device. Incorrect NIC drivers, interface cards, and hardware problems with switches can cause Layer 2 problems to occur.

Step 5: Layer 1 converts the data to bits for transmission.
The frame is converted into a pattern of 1s and 0s (bits) for transmission on the medium. A clocking function enables the devices to distinguish these bits as they travel across the medium. The medium can change along the path between the source and destination. For example, the email message can originate on an Ethernet LAN, cross a fiber campus backbone, and cross a serial WAN link until it reaches its destination on another remote Ethernet LAN. Layer 1 problems can be caused by loose or incorrect cables, malfunctioning interface cards, or electrical interference.

At the receiving host, the processes described in steps 1 through 5 are reversed, with the message traveling back up the layers to the appropriate application.

Troubleshooting the OSI Model

As a theoretical model, the OSI model defines the protocols, hardware, and other specifications that operate at the seven layers.

The OSI model also provides a systematic basis for troubleshooting a network. In any troubleshooting scenario, the basic problem-solving procedure includes the following steps:
1. Define the problem.
2. Isolate the cause of the problem.
3. Solve the problem.
– Identify and prioritize alternative solutions.
– Select one alternative as the solution.
– Implement the solution.
– Evaluate the solution.

If an identified solution does not fix the problem, undo any changes and proceed to the next possible solution. Go through the steps until a solution works.

In addition to the basic problem-solving procedures, the OSI model can be used as a guideline for troubleshooting. Using a layered model, there are three different troubleshooting approaches that a technician can use to isolate the problem:
Bottom-Up – The bottom-up approach starts with the physical components of the network and works its way up the layers of the OSI model. Bottom-up troubleshooting is an effective and efficient approach for suspected physical problems.
Top-Down – The top-down approach starts with the user application and works its way down the layers of the OSI model. This approach starts with the assumption that the problem is with the application and not the network infrastructure.
Divide-and-Conquer – The divide-and-conquer approach is generally used by more experienced network technicians. The technician makes an educated guess targeting the problem layer and then based on the observed results, moves up or down the OSI layers.

Using the OSI model as a guide, the help desk technician can query the customer to help define the problem and isolate the cause.

Layer 1 Troubleshooting
The technician starts with Layer 1 issues first. Remember, Layer 1 deals with the physical connectivity of the network devices. Layer 1 problems often involve cabling and electricity, and are the reasons for many help desk calls. Some of the more common Layer 1 problems include:
– Device power turned off
– Device power unplugged
– Loose network cable connection
– Incorrect cable type
– Faulty network cable
– Faulty wireless access point
– Incorrect wireless settings, such as the SSID
When remotely troubleshooting a problem, the technician should advise the caller through each step, what to look for, and what to do if an error is found. If it is determined that all Layer 1 issues have been addressed, it is time to travel up the OSI model to Layer 2.

Layer 2 Troubleshooting
Network switches and host NICs perform Layer 2 functions. Layer 2 problems can be caused by faulty equipment, incorrect device drivers, or an improperly configured switch. When remotely troubleshooting a problem, it may be difficult to isolate a Layer 2 problem.
An on-site technician can check whether the NIC is installed and working properly. Reseating the NIC, or replacing a suspected faulty NIC with a known good NIC, helps to isolate the problem. The same process can be done with any network switch.

Layer 3 Troubleshooting
At Layer 3, the technician needs to investigate the logical addressing used in the network, such as the IP address scheme. If the network is using IP addressing, the technician verifies that the device has the proper settings, such as:
– IP address within the assigned network
– Correct subnet mask
– Correct default gateway
– Other settings as required, such as DHCP or DNS
At Layer 3, several utilities can assist with the troubleshooting process. Three of the most common command line tools are:
ipconfig – Shows IP settings on the computer
ping – Tests basic network connectivity
tracert – Determines if the routing path between the source and destination is available
Most network problems can usually be resolved using these Layer 1, 2, and 3 troubleshooting techniques.

Layer 4 Troubleshooting
If Layers 1 through 3 all appear to be operating normally and the technician can successfully ping the IP address of the remote server, it is time to check the higher layers. For example, if a network firewall is used along the path, it is important to check that the application TCP or UDP port is open and no filter lists are blocking traffic to that port.

Layers 5 through 7 Troubleshooting
The technician should also check the application configuration. For example, if troubleshooting an email issue, ensure that the application is configured with the correct sending and receiving email server information. It is also necessary to ensure that domain name resolution is functioning as expected.
For remote technicians, higher layer issues can be checked by using other network utility tools, such as a packet sniffer, to view traffic as it crosses the network. A network application, such as Telnet, can also be used to view configurations.

Help Desk Troubleshooting Scenarios

The number and types of calls received by the help desk can vary extensively. Some of the most common calls include problems with email, host configuration, and connectivity.

Email Issues
– Can receive but not send
– Can send but not receive
– Cannot send or receive
– Nobody can reply to messages

A common cause of many email problems is using the wrong POP, IMAP, or SMTP server names. It is best to check with the email administrator to confirm the proper name of the POP or IMAP server and SMTP server. In some cases, the same server name for both POP/IMAP and SMTP are used. Also, confirm that the username and password are correct. Since the password is not usually displayed, it is a good idea to carefully re-enter it.

Host Configuration Issues
A common issue that can prevent connectivity to the Internet or other network resources is improperly configured host addressing information. This can include an incorrect IP address, subnet mask, or default gateway.
In environments where the IP addressing information is manually configured, it is possible that the IP configuration was simply entered incorrectly. In environments where hosts are configured to dynamically receive an IP address from an assignment server, such as a DHCP server, the server may fail or become unreachable due to network issues.
If a host is configured to receive an address dynamically, and an assignment server is unavailable or unreachable, a link-local address will be automatically assigned to the local host by the operating system. IPv4 addresses in the address block to ( /16) are designated as link-local addresses. A link-local process will randomly select an IP address within the range. But what prevents two hosts from randomly selecting the same IP address?
Once the link-local process selects an IP address, it sends an ARP query with that IP onto the network to see if any other devices are using that address. If there is no response, the IP address is assigned to the device, otherwise another IP address is selected, and the ARP query is repeated. Microsoft refers to link-local addresses as Automatic Private IP Addressing (APIPA).
If multiple hosts on the same network obtain a link-local address, client/server and peer-to-peer applications between those hosts will work properly. However, because link-local addresses are in the private Class B address space, communication outside of the local network is not possible.
When troubleshooting both manually and dynamically configured hosts, use the host command ipconfig /all to verify that the host is using the appropriate IP configuration.

Customer Connectivity Issues
Connectivity problems are more common with new customers trying to connect for the first time. However, sometimes existing customers encounter connectivity issues. First-time customers may have problems with installing the hardware as well as software configuration settings. Existing customers notice connectivity problems when they cannot open a web page or connect to instant messaging or email.
There are many reasons why a customer has no connectivity, including the following:
– Delinquent payments for services
– Hardware failures
– Physical layer failures
– Incorrect application settings
– Missing application plug-ins
– Missing applications
In many cases, the problem is simply a faulty cable, or a cable plugged into an incorrect port. These types of issues can be resolved by checking the cable connection or replacing the cable.

Other problems, such as software issues, may be more difficult to detect. One example is an incorrectly loaded TCP/IP stack, preventing IP from operating correctly. The TCP/IP stack can be tested and verified using a loopback address. The loopback is a special address, the reserved IPv4 address, which hosts use to direct traffic to themselves. The loopback address creates a shortcut method for TCP/IP applications and services that run on the same device to communicate.

Creating and Using Help Desk Records

When a Level 1 help desk technician receives a call, there is a process followed to gather information. There are also specific systems for storing and retrieving relevant information. It is extremely important to gather the information correctly in the event that a call has to be escalated to Layer 2 or require an on-site visit.
The information gathering and recording process starts as soon as the technician answers the phone. When the customer identifies who they are, the technician accesses the relevant customer information. Typically, a database application is used to manage the customer information.

The information is transferred to a trouble ticket, or incident report. This document can be a piece of paper in a paper filing system or an electronic tracking system designed to follow the troubleshooting process from beginning to end. Each person who works on the problem is expected to record what was done on the trouble ticket. When an on-site call is required, the trouble ticket information can be converted to a work order that the on-site technician can take to the customer site.
When a problem is resolved, the solution is documented in the customer work order or trouble ticket, and in a knowledge-base document for future reference.

Occasionally, the Level 1help desk technician may receive a call that cannot be resolved quickly. In this instance, the technician is responsible for passing the call to a Level 2 technician who is more qualified to resolve the issue. Passing the call to a higher level technician is known as the call escalation process.
Both Level 1 and Level 2 help desk technicians attempt to solve customer problems using the telephone, web tools, and possibly remote desktop sharing applications.

If the help desk technicians are not able to fix the problem remotely, it is often necessary to send a Level 3 on-site technician to the customer premise location. It is the job of the on-site technician to visit the customer premise to physically work on the problem equipment. The help desk technician can make an appointment with the customer for the on-site technician to perform the repairs, or it may be the responsibility of the on-site technician to arrange the appointment.

To properly troubleshoot the problem, the on-site technician reviews the trouble ticket to see what was previously done. This review gives the technician some background information and a logical starting point. It also helps the technician decide which tools and supplies to bring, rather than having to leave the customer site to obtain supplies.
On-site technicians typically work on the network at the customer location, although there are instances where the technician is unable to make the needed repairs and must bring the damaged equipment back to the ISP site for additional troubleshooting.

Customer Site Procedures

There are four steps an on-site technician performs before beginning any troubleshooting or repair at the customer site:

– Step 1. Provide proper identification to the customer.
– Step 2. Review the trouble ticket or work order with the customer to verify that the information is correct.
– Step 3. Communicate the current status of any identified problems and the actions the technician expects to take at the customer site that day.
– Step 4. Obtain permission from the customer to begin the work.

The technician must verify all items on the trouble ticket. Once the technician is familiar with all issues, the work can begin. The technician is responsible for checking all device and network settings, and running any necessary utilities. The technician may also have to swap out suspected faulty hardware with known good hardware to determine if a hardware problem exists.

When performing any troubleshooting tasks the customer site, especially when installing new or replacing existing equipment, it is important to minimize the risk of injury by following good safety practices. Many employers offer safety training as part of their employee services.
Use ladders to reach high locations to install networking cable and to install or troubleshoot wireless access points in places that are difficult to reach. To reduce the risk of falling off the ladder or dropping equipment while climbing on the ladder, work with a partner whenever possible.
High or Dangerous Locations
Sometimes network equipment and cables are located in high and dangerous places, such as on the side of a building, on roof tops, or in an internal structure such as an elevator shaft, that is not accessible by a ladder. Work performed at this type of location must be done very carefully. Using a safety harness reduces the risk of falling.
Electrical Equipment
If there is a risk of damaging or coming in contact with any electrical lines when handling hardware, consult with the electrician of the customer about measures that can be taken to reduce the risk of electrical shock. Coming in contact with electrical equipment may result in serious personal injury.
Awkward Spaces
Network equipment is often located in narrow and awkward spaces. Ensure that the work area is properly lighted and ventilated. Determine the best way to lift, install, and remove equipment to minimize the risks.
Heavy Equipment
Networking devices can be large and heavy. Plan to have the correct equipment and trained personnel when heavy equipment needs to be installed or moved at a customer site.

After the technician makes any configuration changes or installs new equipment, the technician must observe the results to ensure proper operation. When finished, the technician communicates the nature of the identified problem to the customer, what solution was applied, and any follow-up procedures. Before the problem can be considered fully resolved, the technician must obtain the acceptance of the customer. The technician can then close the trouble ticket and document the solution.

A copy of the documentation is left with the customer. The document includes the original help desk call problem and the actions taken to solve the problem. The technician records the solution, and the customer acceptance is indicated on the trouble ticket. For future reference, the technician also records the problem and the solution in the help desk documentation and FAQs.

In some cases, an on-site technician can uncover network problems that require upgrades or reconfiguration of the network devices. When this occurs, it may be outside of the scope of the original trouble ticket. These issues are usually communicated to both the customer and the ISP network personnel for further action.

October 6, 2010

Chapter 1 – The Internet and Its Uses

Filed under: Uncategorized — krystalchisholm @ 9:52 am

The Internet

The internet is a large system of interconnecting networks allowing worldwide access to an almost infinite amount of information. It allows businesses and individuals alike to communicate over long distances and share information, resources and services.

In the beginning, the internet was used almost strictly for scientific, military and educational uses. As time went on, it expanded and allowed businesses and consumers to access the system. As time went on and new technologies are developed, the internet has grown, and will continue to grow on into the future. There are a variety of online applications for people to use; like email, web browsing, streaming music and videos, gaming, and instant messaging.

The way people interact, share information, and even do business is changing to keep up with the continuous evolution of this global network. The Internet is creating a wider audience and consumer base for whatever message, product, or service can be delivered. For many businesses, having Internet access has become critical, not only for communication but also for day-to-day operation. Some of the business uses of the Internet include:
– E-Commerce
– Communications
– Collaboration and training

Internet Standards

With the internet continuing to grow, and new technologies being developed, there is also developed a set of rules so these devices can all interact over the internet. These are the internet standards.
One person can send their friend an email from her home desktop. The friend can access this email from his smart phone, so long as the phone adheres to the same standards as the home desktop.

An Internet standard is the end result of a comprehensive cycle of discussion, problem solving, and testing. When a new standard is proposed, each stage of the development and approval process is recorded in a numbered Request for Comments (RFC) document so that the evolution of the standard is tracked.

As the number of standards increase, it also increases the number of items that can access the internet. There are now thousands of internet standards, and this means almost any electronic device has the ability to access the internet through various means. This includes mobile phones, PDAs, MP3 players, televisions, computers, even fridges.

ISP and ISP Services

No matter what the device is, there is one crucial thing that they need to access the internet, and that is a connection to an Internet Service Provider (ISP). An ISP is a company or organization through which a subscriber obtains Internet access. A subscriber can be a business, a private consumer, a government body, or even another ISP.

In addition to offering connection to the Internet, an ISP can offer other services to subscribers, including:
Equipment co-location – A business may opt to have some or all internal network equipment physically located on the ISP premises.
Web hosting – The ISP provides the server and application software for storing web pages and web content for the business website.
FTP – The ISP provides the server and application software for the FTP site of a business.
Applications and media hosting – The ISP provides the server and software to allow a business to provide streaming media such as music, video, or applications such as online databases.
Voice over IP – A business can save on long distance telephone charges, especially for internal calls between geographically distant offices, by using Voice over IP (VoIP).
Technical support – Many businesses do not have the in-house technical expertise to manage large internal networks. Some ISPs provide technical support and consulting services for an additional fee.
Point of Presence (POP) – A business has the option of connecting to the ISP through POP, using a variety of access technologies.

Delivering Internet Service to End Users

To gain access to the Internet, it is first necessary to have a connection to an ISP. ISPs offer various connection options. The main connection methods used by home and small business users are:
Dialup access
Dialup access is an inexpensive option that uses any phone line and a modem. To connect to the ISP, a user calls the ISP access phone number. Dialup is the slowest connection option, and is typically used by mobile workers and in areas where higher speed connection options are not available.
Digital subscriber line, or DSL, is more expensive than dialup, but provides a faster connection. DSL also uses telephone lines, but unlike dialup access, DSL provides a continuous connection to the Internet. This connection option uses a special high-speed modem that separates the DSL signal from the telephone signal and provides an Ethernet connection to a host computer or LAN.
Cable modem
A cable modem is a connection option offered by cable television service providers. The Internet signal is carried on the same coaxial cable that delivers cable television to homes and businesses. A special cable modem separates the Internet signal from the other signals carried on the cable and provides an Ethernet connection to a host computer or LAN.
Satellite connection is an option offered by satellite service providers. The user’s computer connects through Ethernet to a satellite modem that transmits radio signals to the nearest Point of Presence, or POP, within the satellite network.

Bandwidth is measured in bits per second (bps). Higher bandwidth speeds are measured in kilobits per second (kbps), megabits per second (Mbps), or gigabits per second (Gbps).
There are three main types of high-bandwidth connection options that are used by businesses:
T1 connections transmit data up to 1.544 Mbps. T1 connections are symmetrical, meaning that the upload bandwidth is the same as the download bandwidth. A medium-sized business may need only one T1 connection. E1 is a European standard that transmits data at 2.048 Mbps.
T3 connections transmit data up to 45 Mbps. Although considerably more expensive than a T1 connection, larger businesses may need a T3 connection to accommodate the number of employees. Large businesses with multiple locations might use a combination of T1 and T3 lines. E3 is a European standard that transmits data at 34.368 Mbps.
Metro Ethernet offers a wide range of high-bandwidth options, including Gbps links. Large companies with many branches in the same city, such as banks, use Metro Ethernet. Metro Ethernet connects the main office location and all the branches using switched technology. Metro Ethernet allows the transfer of large amounts of data faster and less expensively than other high-bandwidth connection options.

After the type of connection is established, it is necessary to connect to the ISP to get access to the Internet. Individual computers and business networks connect to the ISP at the POP. POPs are located at the edge of the ISP network and serve a particular geographical region. They provide a local point of connection and authentication (password control) for multiple end users. An ISP may have many POPs, depending on the size of the POP and the area that it services.
Within the ISP network, high-speed routers and switches move data between the various POPs. Multiple links interconnect the POPs to provide alternate routes in case one of the links becomes overloaded with traffic or fails.

Internet Hierarchy

The Internet has a hierarchical structure. At the top of this hierarchy are the ISP organizations. The ISP POPs connect to an Internet Exchange Point (IXP). In some countries, this is called a Network Access Point (NAP). An IXP or NAP is where multiple ISPs join together to gain access to each other’s networks and exchange information. There are currently over 100 major exchange points located worldwide.
The Internet backbone consists of this group of networks owned by various organizations and interconnected through IXPs and private peering connections.
The Internet backbone is like an information super highway that provides high-speed data links to interconnect the POPs and IXPs in major metropolitan areas around the world. The primary medium that connects the Internet backbone is fiber-optic cable. This cable is typically installed underground to connect cities within continents. Fiber-optic cables also run under the sea to connect continents, countries, and cities.

ISPs are classified into different tiers according to how they access the Internet backbone:
Tier 1 ISPs are the top of the hierarchy. Tier 1 ISPs are huge organizations that connect directly with each other through private peering, physically joining their individual network backbones together to create the global Internet backbone. Within their own networks, the Tier 1 ISPs own the routers, high-speed data links, and other pieces of equipment that join them to other Tier 1 ISP networks. This includes the undersea cables that connect the continents.
Tier 2 ISPs are the next tier in terms of backbone access. Tier 2 ISPs can also be very large, even extending across several countries, but very few have networks that span entire continents or between continents. To provide their customers with global Internet access, some Tier 2 ISPs pay Tier 1 ISPs to carry their traffic to other parts of the world. Some Tier 2 ISPs exchange global traffic with other ISPs less expensively through public peering at IXPs. A large IXP may bring together hundreds of ISPs in a central physical location for access to multiple networks over a shared connection.
Tier 3 ISPs are the farthest away from the backbone. Tier 3 ISPs are generally found in major cities and provide customers local access to the Internet. Tier 3 ISPs pay Tier 1 and 2 ISPs for access to the global Internet and Internet services.

Using Tools to Map the Internet

Network utilities create a map of the various interconnections to visualize how ISP networks interconnect. These utilities also illustrate the speed at which each connecting point can be reached.

The ping command tests the accessibility of a specific IP address. The ping command sends an ICMP (Internet Control Message Protocol) echo request packet to the destination address and then waits for an echo reply packet to return from that host. ICMP is an Internet protocol that is used to verify communications. It measures the time that elapses between when the request packet is sent and the response packet is received. The ping command output indicates whether the reply was received successfully and displays the round-trip time for the transmissions.

If a packet does not reach the destination, or if delays are encountered along the way, how is it determined where the problem is located or through which routers the packet has passed?

The traceroute utility displays the path that a packet takes from the source to the destination host. Each router that the packet passes through is called a hop. Traceroute displays each hop along the way. It also calculates the time between when the packet is sent and when a reply is received from the router at each hop.

If a problem occurs, use the output of the traceroute utility to help determine where a packet was lost or delayed. The output also shows the various ISP organizations that the packet must pass through during its journey from source to destination.
The Windows tracert utility works the same way. There are also a number of visual traceroute programs that provide a graphical display of the route that a packet takes.

ISP Requirements

An ISP requires a variety of devices to accept input from end users and provide services. To participate in a transport network, the ISP must be able to connect to other ISPs. An ISP must also be able to handle large volumes of traffic. Some of the devices required to provide services include:
– Access devices that enable end users to connect to the ISP, such as a DSL Access Multiplexer (DSLAM) for DSL connections, a Cable Modem Termination System (CMTS) for cable connections, modems for dialup connections, or wireless bridging equipment for wireless access.
– Border gateway routers to enable the ISP to connect and transfer data to other ISPs, IXPs, or large business enterprise customers.
– Servers for such things as email, network address assignment, web space, FTP hosting, and multimedia hosting.
– Power conditioning equipment with substantial battery backup to maintain continuity if the main power grid fails.
– High capacity air conditioning units to maintain controlled temperatures.

ISPs, like other businesses, want to expand so that they can increase their income. The ability to expand their business depends on gaining new subscribers and selling more services. However, as the number of subscribers grows, the traffic on the network of the ISP also grows.

Eventually, the increased traffic may overload the network, causing router errors, lost packets, and excessive delays. In an overloaded network, subscribers can wait for minutes for a web page to load, or may even lose network connection. These customers may choose to switch to a competing ISP to get better performance.
Loss of customers directly translates to loss of income for an ISP. For this reason, it is important that the ISP provides a reliable and scalable network.
Scalability is the capacity of a network to allow for future change and growth. Scalable networks can expand quickly to support new users and applications without affecting the performance of the service being delivered to existing users.
The most scalable devices are those that are modular and provide expansion slots for adding modules. Different modules can have different numbers of ports. In the case of a chassis router, some modules also offer different interface options, allowing for different connection options on the same chassis.

Roles and Responsibilities Within the ISP

ISPs are organisations like any other. They have people in multiple teams and departments, each with their own responsibilities. For an ISP, this is mostly in ensuring that the network stays up and running for the customers.
Network support services are involved in all aspects of network management, including planning and provisioning of new equipment and circuits, adding new subscribers, network repair and maintenance, and customer service for network connectivity issues.
When a new business subscriber orders ISP services, the various network support service teams work together to ensure that the order is processed correctly and that the network is ready to deliver those services as quickly as possible.
Each of the network support service teams have their own roles and responsibilities:
– Customer Service receives the order from the customer and ensures that the specified requirements of the customer are accurately entered into the order tracking database.
– Planning and Provisioning determines whether the new customer has existing network hardware and circuits and if new circuits need to be installed.
– The On-site Installation is advised of which circuits and equipment to use and then installs them at the customer site.
– The Network Operations Center (NOC) monitors and tests the new connection and ensures that it is performing properly.
– The Help Desk is notified by the NOC when the circuit is ready for operation and then contacts the customer to guide them through the process of setting up passwords and other necessary account information.

September 12, 2010

Chapter 9 – Network Troubleshooting

Filed under: Uncategorized — krystalchisholm @ 8:32 pm

Troubleshooting is the process of identifying, locating and correcting problems that occur. Experienced individuals often rely on instinct to troubleshoot. However, there are structured techniques that can be used to determine the most probable cause and solution.
When troubleshooting, proper documentation must be maintained. This documentation should include as much information as possible about:
– The problem encountered
– Steps taken to determine the cause of the problem
– Steps to correct the problem and ensure that it will not reoccur
Document all steps taken in troubleshooting, even the ones that did not solve the issue. This documentation becomes a valuable reference should the same or similar problem occur again.

Gathering information
One of the first ways to gather information is to question the individual who reported the problem as well as any other affected users. Questions can include: end user experiences, observed symptoms, error messages and information about recent configuration changes to devices or applications.
Next, collect information about any equipment that may be affected. This can be gathered from documentation. A copy of all log files and a listing of any recent changes made to equipment configurations is also necessary. Other information on the equipment includes the manufacturer, make and model of devices affected, as well as ownership and warranty information. The version of any firmware or software on the device is also important because there may be compatibility problems with particular hardware platforms.
Information about the network can also be gathered using network monitoring tools. Network monitoring tools are complex applications often used on large networks to continually gather information about the state of the network and network devices. These tools may not be available for smaller networks.

There are several different structured troubleshooting techniques available, including:
– Top-down
– Bottom-up
– Divide-and-conquer

Top-down starts with the application layer and works down. It looks at the problem from the point of view of the user and the application. Is it just one application that is not functioning, or do all applications fail? For example, can the user access various web pages on the Internet, but not email? Do other workstations have similar issues?

Bottom-up starts with the physical layer and works up. The physical layer is concerned with hardware and wire connections. Have cables been pulled out of their sockets? If the equipment has indicator lights, are they on or off?

Divide-and-Conquer typically begins troubleshooting at one of the middle layers and works up or down from there. For example, the troubleshooter may begin at the network layer, by verifying IP configuration information.

Trial and Error
Trial and error relies on individual knowledge to determine the most probable cause of a problem. A troubleshooter makes an educated guess on the most likely solution based on past experience and knowledge of the network structure. Once the solution is implemented, if it does not work, the troubleshooter uses this information to help determine the next most likely cause. This process is repeated until the problem is isolated and solved.
With this technique the problem is assumed to be caused by a specific hardware component or a configuration file. The defective part or code is replaced by a known good device or file. While not necessarily locating the problem, this technique can save time and quickly restore network functionality. This relies on the availability of substitute parts, components, and backup configuration files which can be very expensive to maintain.

A number of software utility programs are available that can help identify network problems. Most of these utilities are provided by the operating system as command line interface (CLI) commands. The syntax for the commands may vary between operating systems.
Some of the available utilities include:
ipconfig – Displays IP configuration information
ping – Tests connections to other IP hosts
tracert – Displays route taken to destination
netstat – Displays network connections
nslookup – Directly queries the name server for information on a destination domain

Ipconfig is used to display the current IP configuration information for a host. Issuing this command from the command prompt will display the basic configuration information including: IP address, subnet mask and default gateway.
Ipconfig /all

The command ipconfig /all displays additional information including the MAC address, IP addresses of the default gateway and the DNS servers. It also indicates if DHCP is enabled, the DHCP server address and lease information. How can this utility assist in the troubleshooting process? Without an appropriate IP configuration, a host can not participate in communications on a network. If the host does not know the location of the DNS servers it cannot translate names into IP addresses.
Ipconfig /release and ipconfig /renew
If IP addressing information is assigned dynamically, the command ipconfig /release will release the current DHCP bindings. Ipconfig /renew will request fresh configuration information from the DHCP server. A host may contain faulty or outdated IP configuration information and a simple renewal of this information is all that is required to regain connectivity.
If the IP configuration appears to be correctly configured on the local host, next, test network connectivity by using ping. Ping is used to test if a destination host is reachable. The ping command can be followed by either an IP address or the name of a destination host. When a ping is sent to an IP address, a packet known as an echo request is sent across the network to the IP address specified. If the destination host receives the echo request, it responds with a packet known as an echo reply. If the source receives the echo reply, connectivity is verified.
If a ping is sent to a name, such as, a packet is first sent to a DNS server to resolve the name to an IP address. Once the IP address is obtained, the echo request is forwarded to the IP address and the process proceeds. If a ping to the IP address succeeds, but a ping to the name does not, there is most likely a problem with DNS. The basic ping command usually issues four echoes and waits for the replies to each one. It can, however, be modified to increase its usefulness.
The Tracert utility provides connectivity information about the path a packet takes to reach the destination and about every router (hop) along the way. It also indicates how long a packet takes to get from the source to each hop and back (round trip time). Tracert can help identify where a packet may have been lost or delayed due to bottlenecks or slowdowns in the network.
The basic tracert utility will only allow up to 30 hops between a source and destination device before it assumes that the destination is unreachable. This number is adjustable by using the -h parameter.
Sometimes it is necessary to know which active TCP connections are open and running on a networked host. Netstat is an important network utility that can be used to verify those connections. Netstat lists the protocol in use, the local address and port number, the foreign address and port number, and the state of the connection.
Unexplained TCP connections can pose a major security threat. This is because they can indicate that something or someone is connected to the local host. Additionally, unnecessary TCP connections can consume valuable system resources thus slowing down the host’s performance. Netstat should be used to examine the open connections on a host when performance appears to be compromised.
Other modifiers, displayed as Options in the graphic, are also available.
When accessing applications or services across the network, individuals usually rely on the DNS name instead of the IP address. When a request is sent to that name, the host must first contact the DNS server to resolve the name to the corresponding IP. The host then uses IP to package the information for delivery.
The nslookup utility allows an end-user to look up information about a particular DNS name in the DNS server. When the nslookup command is issued, the information returned includes the IP address of the DNS server being used as well as the IP address associated with the specified DNS name. Nslookup is often used as a troubleshooting tool for determining if the DNS server is performing name resolution as expected.

Connectivity problems occur on wireless networks, wired networks and networks that use both. When troubleshooting a network with both wired and wireless connections, it is often best to troubleshoot using a divide-and -conquer technique to isolate the problem to either the wired or wireless network. The easiest way to determine if the problem is with the wired or the wireless network is to:
1. Ping from a wireless client to the default gateway – this verifies if the wireless client is connecting as expected.
2. Ping from a wired client to the default gateway – this verifies if the wired client is connecting as expected.
3. Ping from the wireless client to a wired client – this verifies if the integrated router is functioning as expected.
Once the problem is isolated it can be corrected.

LED Indicators
Three types of LEDs are commonly found on devices – power, status and activity. On some devices a single LED may convey multiple pieces of information depending on the current status of the device. It is important to check the equipment documentation for the exact meaning of all indicators but some commonality does exist.
Inactive LEDs may be an indication of a device failure, port failure, or cabling issues. It is possible that the device is non-functional due to faulty hardware. The port itself might also have become faulty due to hardware or improperly configured software. Regardless of whether the network is wired or wireless, verify that the device and ports are up and functional before spending large amounts of time trying to troubleshoot other issues.

Wired host cannot connect to the integrated router
If the wired client is unable to connect to the integrated router, one of the first things to check is the physical connectivity and cabling. Cabling is the central nervous system of wired networks and one of the most common issues when experiencing inactivity.
There are several issues to watch for in cabling:
1. Be sure to use the correct type of cable. Two types of UTP cables are commonly encountered in networking: Straight-through cables and Cross-over cables. Using the wrong type of cable may prevent connectivity.
2. Improper cable termination is one of the main problems encountered in networks. To avoid this, cables should be terminated according to standards.
– Terminate cables via 568A or 568B termination standard
– Avoid untwisting too much cable during termination
– Crimp connectors on the cable jacket to provide strain relief
3. Maximum cable run lengths exist based on characteristics of the different cables. Exceeding these run lengths can have a serious negative impact on network performance.
4. If connectivity is a problem, verify that the correct ports are being used between the networking devices.
5. Protect cables and connectors from physical damage. Support cables to prevent strain on connectors and run cable through areas that will not be in the way.

Wireless host cannot connect to the AP
If the wireless client is unable to connect to the AP, it may be because of wireless connectivity problems. Wireless communications rely on radio frequency signals (RF) to carry data. Many factors can affect our ability to connect hosts using RF.
1. Not all wireless standards are compatible. The 802.11a (5 GHz band) is not compatible with the 802.11b/g/n standards (2.4 GHz band). Within the 2.4 GHz band, each standard uses different technology. Unless specifically configured, equipment that conforms to one standard may not function with that conforming to another.
2. Each wireless conversation must occur on a separate, non-overlapping channel. Some AP devices can be configured to select the least congested or highest throughput channel. Although automatic settings work, manual setting of the AP channel provides greater control and may be necessary in some environments.
3. The strength of an RF signal decreases with distance. If the signal strength is too low, devices will be unable to reliably associate and move data. The signal may be dropped. The NIC client utility can be used to display the signal strength and connection quality.
4. RF signals are susceptible to interference from outside sources, including other devices functioning on the same frequency. A site survey should be used to detect for this.
5. APs share the available bandwidth between devices. As more devices associate with the AP, the bandwidth for each individual device will decrease causing network performance problems. The solution is to reduce the number of wireless clients using each channel.

Wireless configuration problems
Modern WLANs incorporate various technologies to help secure the data on the WLAN: incorrect configuration of any of these can prevent communication. Some of the most common settings that are configured incorrectly include: the SSID, authentication and encryption.
1. The SSID is a case-sensitive, alpha-numeric string up to 32-characters. It must match on both the AP and client. If the SSID is broadcast and detected, this is not an issue. If the SSID is not broadcast, it must be manually entered onto the client. If the client is configured with the wrong SSID, it will not associate with the AP. Additionally, if another AP is present that is broadcasting the SSID, the client may automatically associate to it.
2. On most APs open authentication is configured by default, allowing all devices to connect. If a more secure form of authentication is configured, a key is necessary. Both the client and the AP must be configured with the same key. If the keys do not match, authentication will fail and the devices will not associate.

When troubleshooting, documentation should be maintained during the troubleshooting process. This documentation can be a valuable reference and can be used when future issues arise. Good troubleshooting documentation should include:
– Initial problem
– Steps taken to isolate the problem
– Results of all steps taken, both successful and unsuccessful
– Final determined cause of the problem
– Final problem resolution
– Preventative measures

If, during the troubleshooting process, the troubleshooter is unable to determine the problem and its resolution, it might be necessary to obtain assistance from outside sources. Some of the most common sources for help include:
– Previously kept documentation
– Online FAQs (Frequently Asked Questions)
– Colleagues and other network professionals
– Internet forums

The helpdesk will require information specific to the problem including:
– Symptoms encountered
– Who encountered the problem
– When the problem manifests
– Steps taken to identify the problem
– Results of steps taken

Record all information regarding the interaction with the helpdesk, such as:
– Time/date of call
– Name/ID of technician
– Problem reported
– Course of action taken
– Resolution/escalation
– Next steps (follow-up)

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