Optional EIGRP Commands over WANs

14 Mar

Optional EIGRP Commands over WANs
There are always particular design and configuration issues concerning WANs. With WANs, more than at any other point in the network, you are likely to deal with limited resources. Therefore, it is with WAN topologies that you will use the bandwidth and bandwidth-percent commands, because they determine the link resources allocated to EIGRP updates and are used to calculate the metrics assigned to routes.

A perennial concern of network administrators is the amount of bandwidth used for overhead traffic. Administrators want to minimize the amount of network control traffic sent through the network to maximize the bandwidth available for user data. One of the major benefits of both EIGRP and OSPF is that they send as little network traffic as possible. This has the advantages of decreasing the convergence time of the network and ensuring that the network traffic that is sent arrives at the destination.

EIGRP Defaults in Bandwidth Utilization
EIGRP will not use more than 50 percent of the stated bandwidth on a link for its own routing traffic. The bandwidth command used on the interfaces of a Cisco router allows the default settings on links to work as intended, by stating the actual bandwidth of the link. This is often necessary on serial links because the default bandwidth is 1.544 Mbps or a T1. If in reality the link is 56 kbps, it is easy to see how EIGRP could saturate the link. EIGRP allows itself to use up to 50 percent of a
T1 link (772 kbps), far exceeding the real bandwidth of the line. This could mean not only dropping data packets because of congestion but also dropping EIGRP packets. This will cause confusion in the network, not to mention miscalculated routes, retransmission, and user frustration as the network slows.

Other technologies, such as OSPF and SMDS on a Cisco router use the bandwidth value to make decisions. You need to ensure that the bandwidth stated is indeed the speed of the link. When you issue the show interface command, the configured bandwidth of the link will be shown along with a field identifying the load on the line. The load is the amount of traffic sent out of the interface, proportional to the bandwidth of the link, in which the bandwidth is the stated bandwidth and not the actual speed of the physical interface.

NOTE If it is necessary to artificially lower the bandwidth using the bandwidth command, this should be done in consideration of the other network applications.

The bandwidth is a logical construct whose value can have wide-reaching implications on the function of your network. It does not affect the actual speed of the link. In fact, it is practical to configure the bandwidth command only on serial lines, where the speed of the link will vary considerably. The following section provides further guidelines from Cisco on bandwidth configuration.

Rules in Configuring Bandwidth over an NBMA Cloud
EIGRP works well over all WAN environments, including point-to-point and NBMA such as Frame Relay, X25, or ATM. The NBMA topology can include either point-to-point subinterfaces or an NBMA hybrid, which is a combination of point-to-point and multipoint configurations.

Cisco identifies three rules that you should follow when configuring EIGRP over an NBMA cloud:
■ EIGRP traffic should not exceed the committed information rate (CIR) capacity of the virtual circuit (VC).
■ EIGRP’s aggregated traffic over all the VCs should not exceed the access line speed of the interface.
■ The bandwidth allocated to EIGRP on each VC must be the same in both directions.
If you understand and follow these rules, EIGRP works well over the WAN. If you do not take care in the configuration of the WAN, EIGRP can swamp the network.

Configuring Bandwidth over a Multipoint Network
The configuration of the bandwidth command in an NBMA cloud depends on the design of the VCs. If the serial line has many VCs in a multipoint configuration, EIGRP will evenly distribute its overhead between the VCs, without the use of subinterfaces. The bandwidth command should therefore reflect the access link speed into the Frame Relay cloud. If the serial interface is accessing an NBMA environment such as Frame Relay, the situation is straightforward. Your company might have five VCs from your router’s serial interface, each carrying 56 kbps. The access link will need a capacity of 5 * 56 kbps. Remember, the aggregate configured bandwidth cannot exceed the access speed of the interface.

Configuring Bandwidth over a Hybrid Multipoint Network
If the multipoint network has differing speeds allocated to the VCs, a more complex solution is needed. There are two main approaches:

■ Take the lowest CIR and simply multiply it by the number of circuits. This is applied to the physical interface. The problem with this configuration is that the higher-bandwidth links will be underutilized for some things.
■ If possible, it is much easier to configure and manage an environment that has used subinterfaces, where a VC is logically treated as if it were a separate interface or point-to point. The bandwidth command can be configured on each subinterface, which allows different speeds on each VC. In this solution, subinterfaces are configured for the links with the differing CIRs. The links that have the same configured CIR are presented as a single subinterface with a bandwidth, which reflects the aggregate CIR of all the circuits. Cisco recommends this as the preferred solution.

The following syntax shows the structure of the bandwidth command:

Router#interface S0
Router(config-if)#bandwidth speed-of-line

Configuring the Pure Point-to-Point Network
If there are many VCs, there might not be enough bandwidth at the access speed of the interface to support the aggregate EIGRP traffic. The subinterfaces should be configured with a bandwidth that is much lower than the real speed of the circuit. In this case, it is necessary to use the bandwidthpercent command to indicate to the EIGRP process that it can still function.

As you learned in the previous section, EIGRP limits itself to 50 percent of the value specified in the bandwidth command, or if the bandwidth command is not set, the interface defaults. If you need to limit this percentage further, the upper limit that EIGRP uses can be stated as a percentage of the bandwidth command. The ip bandwidth-percent-eigrp command interacts with the bandwidth command on the interface. You would use this command primarily because in your network, the bandwidth command does not reflect the true speed of the link. The bandwidth command might have been altered to manipulate the routing metric and path selection of a routing protocol, such as IGRP or OSPF. It might be better to use other methods of controlling the routing metric and return the bandwidth to a true value. Otherwise, the ip bandwidth-percent eigrp command is available. It is possible to set a bandwidth percent that is larger than the stated bandwidth. This is with the understanding that, although the bandwidth might be stated to be 56 kbps, the link is in fact 256 kbps. The following shows the structure of the ip bandwidth-percent eigrp command:

Router(config)#interface S0
Router(config-if)#ip bandwidth-percentt eigrp autonomous-system-number percent

Verifying the EIGRP Operation
Understanding the output of the commands discussed in this section is important, not just because they might constitute questions on the exam, but because they reflect your conceptual understanding of the subject. The ability to analyze what is happening on the network demands a thorough understanding of the concepts explained in this chapter. This skill is required in interpreting the output of a show command.

The ability to interpret these show command output examples in conjunction with the physical and logical topology diagrams of your organization will ensure your understanding of the operation of EIGRP.

This section deals with the show commands shown in Table 14-4.
Table 14-4 EIGRP show Commands

The EIGRP show commands are highly detailed and give a comprehensive understanding of the state of the network. The other commands generic to IP—show ip route and show ip protocols , as described in Chapter 7, “Configuring OSPF in a Single Area”—are also useful in the maintenance of EIGRP.

The show ip eigrp neighbors Command
This show ip eigrp neighbors command shows the neighbor table. The syntax is as follows:

Router#show ip eigrp neighbors [ type number]
Example 14-3 shows the output of this command.
Example 14-1 The show ip eigrp neighbors Output

Table 14-5 explains the meaning of the important fields in Example 14-3.
Table 14-5 Explanation of the show ip eigrp neighbors Command Results

The show ip eigrp topology Command
The show ip eigrp topology command shows the topology table. This command allows for the analysis of DUAL. It shows whether the successor or the route is in an active or passive state. The syntax is as follows:

Router#show ip eigrp topology [ autonomous-system-number | [[ ip-address] mask]]
Example 14-4 shows the output of this command.
Example 14-1 The show ip eigrp topology Output

Table 14-6 explains the meaning of the important fields in Example 14-4.
Table 14-6 Explanation of the show ip eigrp topology Command Results

Table 14-6 Explanation of the show ip eigrp topology Command Results (Continued)

The show ip eigrp traffic command shows the EIGRP traffic received and generated by the router.
The following is the command syntax:
Router#show ip eigrp trafic [ autonomous-system-number]
Example 14-5 shows the output of this command.
Example 14-1 The show ip eigrp traffic Command Output

Troubleshooting the EIGRP Operation
Many methods and tools help in troubleshooting any network. One of the main keys is documentation, for several reasons: For example, you can see progress and easily eliminate the obvious in a checklist manner, and you can clearly explain the problem and the steps taken so far in solving it if you need to call in expert help. Cisco provides many tools both on its web page and in service contracts to help solve your network problems. One of the mainstays in troubleshooting any routing protocol is the group of debug commands, which provide the ability to see traffic and router processes in real time.

Care should be exercised in the use of the debug command, because it can be very greedy in terms of the resources that it consumes. It should be used only for a specific option and for a finite time.
The options available for monitoring EIGRP are covered in Table 14-8.

Table 14-8 The debug Command Options for EIGRP

Foundation Summary
The “Foundation Summary” section of each chapter lists the most important facts from the chapter. Although this section does not list every fact from the chapter that will be on your exam, a wellprepared candidate should, at a minimum, know all the details in each “Foundation Summary” before going to take the exam.

Table 14-9 summarizes the commands covered in this chapter.
Table 14-9 Summary of Commands

Table 14-9 Summary of Commands (Continued)

As mentioned in the introduction, “All About the CCNP, CCDP, and CCIP Certifications,” you have two choices for review questions. The questions that follow next give you a bigger challenge than the exam itself by using an open-ended question format. By reviewing now with this more difficult question format, you can exercise your memory better and prove your conceptual and factual knowledge of this chapter. The answers to these questions are found in Appendix A.

For more practice with examlike question formats, including questions using a router simulator and multichoice questions, use the exam engine on the CD-ROM.

1. Which command has superceded the passive-interface command for preventing EIGRP traffic from traversing a link?
2. What is the preferred configuration for a hybrid multipoint NBMA network when one VC has
a CIR of 56 kbps and the other five VCs each have a CIR of 256 kbps?
3. With four Frame Relay circuits in a multipoint solution and a bandwidth configuration of 224, what is the EIGRP bandwidth allocation per circuit, and where would the bandwidth command be configured?
4. Explain the purpose of the command no auto-summary .
5. Explain the meaning of the command ip bandwidth-percent eigrp 63 100 .
6. In what instances will EIGRP automatically redistribute?
7. How long is the holdtime, by default?
8. For what is the variance command used?
9. What command is used to display the passive and active state of the routes?
10. What command is used in EIGRP to perform manual summarization?
11. For Frame Relay, when would you configure the physical interface (as opposed to a subinterface) with the bandwidth command?
12. Which command is used to display all types of EIGRP packets that are both received and sent by a router?
13. What problems can be solved with the configuration of summarization?
14. Why would you configure an EIGRP router as a stub router?
15. Explain the parameters receive-only , connected , static, and summary used in the command eigrp stub .
16. When configuring the variance command, which routes can be used?
17. Give two reasons why you might wish to change the Hello timer.
18. What should be considered in terms of EIGRP configuration when there are many VCs entering a WAN and no subinterfaces have been configured?
19. On what occasions should you consider configuring the bandwidth on subinterfaces?

The following scenarios and questions are designed to draw together the content of the chapter and to exercise your understanding of the concepts. There is not necessarily a right answer. The thought process and practice in manipulating the concepts are the goals of this section. The answers to the scenario questions are found at the end of this chapter. The information used in these scenarios was adapted from the Cisco web page, “Cisco Configuration Guidelines.”

Scenario 14-1
The multinational company Gargantuan, Inc., has had a consultant completely readdress the company. The company used the private network and created a hierarchical addressing structure. Refer to Figure 14-3 to see this addressing scheme.

Figure 14-3 Diagram for Scenario 14-1

The addressing of the network was a major project, with all the necessary pitfalls that attend such a large exercise. The network is now stable, and it is time to solve the problems that are being experienced as timeouts and network crashes.

The consultant assured the company that the resolution to the delays was the addressing scheme, butalthough the network is easier to manage, there has been no change in the congestion on the network. In addition, EIGRP appears to be losing routes from its routing tables, which is adding to the problem.

The consultant was correct: The network needed to be readdressed to allow EIGRP to function effectively. Unfortunately, the company did not read the report carefully and missed the other part of the solution.

1. What needs to be done in addition to solve the addressing problems that make EIGRP inefficient? Give the configuration commands necessary to activate this solution on Router A.

The WAN is a Frame Relay cloud, and Router A is the hub in the hub-and-spoke configuration. Each VC is 56 kbps.

2. Give the commands to configure Router A for EIGRP over this NBMA cloud.
3. Give the commands to configure Router B for EIGRP over this NBMA cloud.

Scenario 14-2
Given the configuration of EIGRP in Example 14-6, perform the tasks and answer the questions listed. The WAN has light user traffic and is a hub-and-spoke configuration, as shown in Figure 14-4.

Example 14-1 Scenario 14-2 Configuration for Router A

The 256-kbps access line to the hub has 56-kbps access lines to each of ten spoke sites. Each link has a Frame Relay CIR of 56 kbps. The access line to each router reflects the CIR. The access line to the hub router, Router A, is 256 kbps, but the CIR of the hub is the same as its access line.

Figure 14-4 Diagram for Scenario 14-2

From a Frame Relay perspective, a circuit is considered oversubscribed when the sum of CIRs of the remote circuits is higher than the CIR of the hub location. With ten links, each with a CIR of 56 kbps, this circuit is clearly oversubscribed.

1. How much bandwidth has each circuit been allocated? Why was this value chosen by the administrator?
2. What bandwidth utilization is available to EIGRP? Why was this value chosen by the administrator?
3. If Router A fails, what would the effect be on the network?
4. Is summarization possible only on the routers entering the WAN cloud, or is it possible on the networks not shown in the figure but on the other side of the routers? Give reasons for your answers.

Scenario Answers

The answers provided in this section are not necessarily the only possible answers to the questions. The questions are designed to test your knowledge and to give practical exercise in certain key areas This section is intended to test and exercise skills and concepts detailed in the body of this chapter.

If your answer is different, ask yourself whether it follows the tenets explained in the answers provided. Your answer is correct not if it matches the solution provided in the book but rather if it has included the principles of design laid out in the chapter.

In this way, the testing provided in these scenarios is deeper: It examines not only your knowledge but also your understanding and ability to apply that knowledge to problems.

If you do not get the correct answer, refer back to the text and review the subject tested. Be certain to also review your notes on the question to ensure that you understand the principles of the subject.

Scenario 14-1 Answers
1. What needs to be done in addition to solve the problems caused by EIGRP? Give the configuration commands necessary to activate this solution on Router A.

The WAN is a Frame Relay cloud, and Router A is the hub in the hub-and-spoke configuration. Each VC is 56 kbps.

The other solution that the consultant suggested was to perform summarization to limit the query range of the routers. This would prevent the routes in the topology table being SIA, which seriously affects the performance of the network.

The commands required are as follows:

3. Give the commands to configure Router B for EIGRP over this NBMA cloud.
The configuration on Router B is as follows:

Because a maximum of 256 kbps is available, you cannot allow any individual PVC to handle more than 25 kbps (256/10). Note that EIGRP will not use more than 22.5 kbps (90 percent of 25 kbps) on this interface, even though its capacity is 56 kbps. This configuration will not affect user data capacity, which will still be able to use the entire 56 kbps, except when EIGRP is using 22 kbps.

2. What bandwidth utilization is available to EIGRP? Why was this value chosen by the administrator?

Because this data rate is low, and because you do not expect very much user data traffic, you can allow EIGRP to use up to 90 percent of the bandwidth.

3. If Router A fails, what would the effect be on the network?

If Router A fails, there would be no communication between the routers in the WAN because Router A is the hub. Each site would function, but they would all be isolated from each other. The neighbor tables would fail to hear the Hellos from the other routers connecting to the WAN and would time out all routes that they had heard from these routers. The topology table would be updated, and the routers would send updates to all their other neighbors.

4. Is summarization possible only on the routers entering the WAN cloud, or is it possible on the networks not shown in the figure, but on the other side of the routers? Give reasons for your answers.

Summarization is possible on all interfaces in EIGRP, as long as the addressing scheme allows for it to be implemented. This is one of the major advantages of EIGRP over OSPF. OSPF can summarize only at ABRs.


No comments yet

Leave a Reply

You must be logged in to post a comment.