A Working Configuration of OSPF on a Multiarea Network

12 Mar

A Working Configuration of OSPF on a Multiarea Network
Example 9-8 is a working configuration tested for verification. It includes many of the commands explained earlier in this chapter. Here you see an entire working configuration rather than the relevant segment for configuring a particular networking nuance. Use the configuration in conjunction with Figure 9-8.

Example 9-8 Configuring OSPF in a Multiarea Network on Router A

Multiarea Network

CAUTION The ip directed-broadcasts command allows broadcasts to be sent across the network as a unicast and to be released as a broadcast on the interface for which it is destined. Although this is a very useful option, it has been exploited by hackers and saboteurs. It is therefore recommended that you disable this feature unless it is specifically required.

It is imperative to check any configuration on a network device because any errors could potentially bring down the entire network. To verify the configuration, a wealth of Cisco commands are available. They are covered in the following section.

Verifying the Configuration of OSPF in a Multiarea Network 309
Figure 9-8 Example 9-8 Network

Example 9-8 Network

Verifying the Configuration of OSPF in a Multiarea Network
The show commands shown here are in addition to the commands described in Chapter 7 in the section “Checking the Configuration of OSPF on a Single Router.” The single router commands are also extremely useful in a multiarea configuration. They are all invaluable in the configuration, troubleshooting, and maintenance of a live network. The following are additional commands that you can use in conjunction with single router commands when verifying OSPF operation on a multiarea network:

■ The show ip ospf border-routers command
■ The show ip route command
■ The show ip ospf database command
■ The show ip ospf virtual-links command

The show ip ospf border-routers Command
The following command shows the OSPF ABRs and ASBRs for which the internal router has entries in its routing table. This command is excellent for troubleshooting configuration errors and understanding how the network is communicating about its routes.

Router#sshhooww iipp ospf border–routers

Example 9-9 shows the output of this command.
Example 9-9 The show ip ospf border-routers Output

border-routers

Table 9-2 explains the meaning of the important fields in the output of the show ip ospf borderrouters command.

The show ip ospf border-routers command is useful to verify that the configuration has worked and that the OSPF network is functioning correctly. In a multiarea network, the show ip ospf border-routers command can immediately indicate why users cannot connect outside their area. This command is helpful in extracting information about the border routers from what could be a long routing table, within which this information is scattered.

Table 9-2 Explanation of the show ip ospf border-routers Command Output

ospf border-routers

Verifying the Configuration of OSPF in a Multiarea Network 311
Table 9-2 Explanation of the show ip ospf border-routers Command Output (Continued)

ospf border-routers

The show ip route Command
The show ip route command is one of the most useful commands available for understanding and troubleshooting an IP network. Example 9-10 shows an example of a routing table in a multiarea network.

Example 9-1 The show ip route Output

show ip route Command

Table 9-3 shows the link-state advertisement (LSA) codes used in the routing table, allowing you to analyze the flow of LSAs both within and between the areas.

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Table 9-3 OSPF Routing Table Codes and Associated LSAs

Across Multiple=

The show ip ospf virtual-links Command The following command shows the configured virtual links that are in existence:

Router#show ip ospf virtual-links

Another command to use in conjunction with this is show ip ospf neighbors , which you learned about in Chapter 7.

Example 9-11 shows the output of the show ip ospf virtual-links command.
Example 9-1 The show ip ospf virtual-links Output

ospf virtual-links

Table 9-4 explains the meaning of the important fields in the output of the show ip ospf virtuallinks command.

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Table 9-4 Explanation of the show ip ospf virtual-links Command

ospf virtual-links

The show ip ospf database Command
The following command shows all the entries in the OSPF link-state database and the information taken from the LSAs that have been received. It can be tailored to show specific information from the database, such as the type 2 LSAs, otherwise known as the networks. To make the output specific to the LSA that you wish to examine, specify the appropriate parameter.

Router#show ip ospf database [router | network|summary|asbr-summary|nssa-external|external|database-summary]

Another command to use in conjunction with this is show ip ospf neighbors .

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Example 9-12 shows the output of the show ip ospf database command.
Example 9-1 The show ip ospf database Output

ip ospf database

Troubleshooting a Multiarea OSPF Network
Troubleshooting OSPF across multiple areas is obviously more complicated than troubleshooting the configuration of a single area. Follow the basic tenets of troubleshooting to aid in unraveling problems and finding solutions in a timely manner:

■ Maintain clear topology maps of the network.
■ Keep current copies of all router configurations.
■ Document changes made to the network.

Following these guidelines is imperative in a complex network to prevent it from becoming a labyrinth from which you cannot escape. The following sections cover the command logadjacency- changes and the most useful debug commands. One of the most common problems that you might experience is the inability of neighbors to form adjacencies. You will also learn about the most common problems that occur in forming adjacencies, the reasons for them, and how to prevent them from occurring.

The log-adjacency-changes Command
A command that has a similar function to the debug command but does not require as many network resources is the log-adjacency-changes command. The debug command provides so much information that it is possible to overrun the logging buffers of the router as well as your own buffers. The log-adjacency-changes command offers less detail, which is often easier to assimilate. Whenever a change in the state of an adjacency occurs, a message is sent to syslog. This means that immediate updates are sent to the administrator without a massive drain on resources.

Router(config-router)#logg–adjacency-changes

Example 9-13 shows the output of the log-adjacency-changes command. In this example, the OSPF process is reconfigured to log adjacency changes. The routing process for OSPF is started and the network command allocates all subnets to Area 0. Whether this is a reconfiguration of the area assignments or OSPF has never been run on this router before, the action is the same. The OSPF process reinitializes all interfaces, requiring the neighbors to be found and the adjacencies formed. With the adjacency changes logged, this example shows the interface Ethernet 0 finding the neighbors at 131.11.84.8 and 131.11.14.14 and all the steps to creating adjacencies being completed.

NOTE In Cisco IOS software release 12.1, the ospf log-adjacency-changes command became log-adjacency-changes.

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Example 9-1 OSPF log-adjacency-changes Output

OSPF Across Multiple=

Useful debug Commands
The debug commands are issued from the interactive console prompt, though remember that you need to be in privileged mode to issue the commands. Any fast-switched packets do not generate messages, so while debug is on, turn off fast switching and force all packets to be process switched for the few minutes that you are using the debug tool.

Use the following debug commands with caution. The resources required by the debug process can overwhelm your system.

The debug ip packet Command
The debug ip packet Command The debug ip packet command is useful for analyzing the messages traveling between the local and remote hosts. IP debugging information includes packets received, generated, and forwarded.

The debug ip ospf events Command
Use the debug ip ospf events to display information on OSPF-related events, such as adjacencies, flooding information, designated router selection, and shortest path first (SPF) calculation.

Common Problems with Adjacencies
Many OSPF problems stem from adjacency problems that propagate throughout the network. Problems are often traced back to neighbor discrepancies. The following guidelines are helpful in these situations.

If a router configured for OSPF routing is not seeing an OSPF neighbor on an attached network, do the following:
■ Make sure that both routers are configured with the same IP mask, MTU, interface Hello timer, OSPF Hello interval, and OSPF dead interval.
■ Make sure that both neighbors are part of the same area and area type.
■ Use the debug and show commands to trace the problem.

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 9-6 shows the codes used in the routing table.
Table 9-6 OSPF Routing Table Codes and Associated LSAs

Codes and Associated LSAs

Table 9-7 contains a list of the commands explained in this chapter. This list is not intended to teach
the use of the commands, but to remind you of the options available.

Table 9-7 OSPF Command Summary

OSPF Command Summary

Q&A
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 in OSPF shows the network LSA information?
2. What command would you use to create a totally stubby area?
3. What is a virtual link, and what command would you use to create it?
4. Where would you issue the command to summarize IP subnets? State the command that is used.
5. How would you summarize external routes before injecting them into the OSPF domain?
6. When is a virtual link used?
7. Give the command for defining the cost of a default route propagated into an area.
8. Give an example of when it would be appropriate to define a default cost.
9. On which router is the area default cost defined?
10. Give the command to configure a stub area and state on which router it is configured.
11. What is the purpose of the area range command, and why is it configured on the ABR?
12. Give the commands to configure a router to place subnets 144.111.248.0 through to
144.111.255.0 in Area 1 and to put all other interfaces into Area 0.
13. Give the syntax to summarize the subnets 144.111.248.0 to 144.111.254.255 into another autonomous system.
14. Explain briefly the difference between the area range command and the summary-address command.
15. Explain the following syntax and what it will achieve: area 1 stub no-summary .
16. Why would you configure the routing process to log adjacency changes as opposed to turning on debug for the same trigger?
17. Give some of the common reasons that neighbors fail to form an adjacency.
18. When configuring a virtual link, which routers are configured?
19. What does the command area 1 default-cost 15 achieve?
20. Explain what is placed in the parameters area-id and router-id for the command area area-id virtual-link router-id.

Scenarios
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 is the goal of this section. The answers to the scenario questions are found at the end of this chapter.

Scenario 9-1
Refer to Figure 9-9 and design the addressing scheme for the network. Then write the configuration for the central router.

Ensure that you include the following:

1. Address the network using the private network 10.0.0.0. Design the addressing scheme so that it allows for the summarization of addresses between areas. Show the summarization that you allocate, and explain your reasons for your choices.

Area 0 is using a prefix of 28 bits within the area.
Area 2 is using a prefix of 22 bits within the area.
Area 3 is using a prefix of 24 bits within the area.
Area 4 is using a prefix of 30 bits for the serial connections. It is using a 28-bit prefix for the connections to the Ethernet routers. Do not include the subnets attached to the LANs in Area 4.

2. Issue the commands for the main router in Figure 9-9 to configure the following:
— The router ID
— The network commands to place the appropriate interfaces into the correct areas
— The configuration of the totally stubby area (Area 3)
— The configuration of the stub (Area 4)
— Summarization between areas
— The election of the central router as designated router, where appropriate

Figure 9-9 The Diagram for Configuration Scenario 9-1

Configuration Scenario

Scenario 9-2
Use Figure 9-10 for this scenario.

The users of the network are complaining about the slowness of the network, particularly when trying to access the Internet. Examine the configuration in Example 9-14 in conjunction with Figure 9-10, and give reasons for any slowness or lack of connectivity that you can see on the network. Provide current configuration commands to correct any errors that you find.

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Figure 9-10 The Diagram for Configuration Scenario 9-2

OSPF Across Multiple=

Example 9-1 Configuring OSPF Example for Scenario 9-2

Configuring OSPF Example for Scenario 9-

Example 9-1 Configuring OSPF Example for Scenario 9-2 (Continued)

Example for Scenario 9-

Answer the questions that follow:
1. There are problems with Router B. There is inconsistency in the routing table, and the system is extremely slow. What commands would be used to identify the problem? In examining the diagram and configuration, what problems can you see?
2. Router A is having problems connecting to Area 0, which is causing problems in other areas because this router is used to connect to Area 0. What commands would be used to identify the problem? In examining the diagram and configuration, what problems can you see?
3. Issue the commands that would be used to correct the configuration problems that you see in the example configuration for Routers A and B.
4. When you issue the show ip ospf interface command, you notice that there is a discrepancy in the timers on the link between Routers A and B. The transmit timer on Router A is set to 5, and the retransmit timer is set to 1. What problems would this cause? What command would be used to change the timers, and what are the default settings?
5. There is an ISDN link into the Internet from Router B. The network manager has suggested that this link is the cause of some performance problems on the router. You have noticed that the interface is included in the OSPF network command. What might be the cause of the problem, and how could it be fixed?

Scenario 9-3
The network administrator is trying to solve a problem in the OSPF network and has determined that the creation of a virtual link is the answer. After studying the figures, answer the associated questions.

1. Explain the purpose of the virtual link in Figure 9-11.
Figure 9-11 Network Diagram 1 for Scenario 9-3

Network Diagram 1 for Scenario

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 9-1 Answers
Refer to Figure 9-9 and design the addressing scheme for the network. Then write the configuration
for the central router.

Table 9-8 shows a possible addressing scheme using the criteria stated in Scenario 9-1. Taking the private address 10.0.0.0, there is a great deal of flexibility in the addressing scheme that can be devised. Remember, however, that careful filtering is required if the organization is to connect to the Internet.

The addressing scheme proposed here is broken out by area. It is not exhaustive in terms of designing an addressing policy down to the LAN level, as was shown in Chapter 3; instead, it deals with the principles of addressing and summarization.

Note in the allocation of addresses that this scenario deals with the allocation of subnets. The addresses displayed in this table are the address ranges of the available subnets, given the prefix length.

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Table 9-8 Allocation of Addresses for Scenario 9-1

Subnet Range

Example 9-1 Sample Configuration of Scenario 9-1 (Continued)

Subnet Range

Scenario 9-2 Answers
1. There are problems with Router B. There is inconsistency in the routing table, and the system
is extremely slow. What commands would be used to identify the problem? In examining the diagram and configuration, what problems can you see?

Router B has been configured to be the designated router for the LAN, which means that it is dealing with all the traffic on the LAN associated with the management of OSPF. Given that the system is a 2500, it is a poor choice for a designated router. A better choice would be Router A, which is a larger system that connects directly to Area 0, making it a better choice from the standpoint of the network design. If Router B were a larger system than a 2500, there could be an argument for making it the designated router to elevate Router A, which would otherwise be functioning as the ABR as well as the designated router.

The router has not been configured as a stub, so the communication between Router A and Router B will be confused, preventing any communication between the two routers.

2. Router A is having problems connecting to Area 0, which is causing problems in other areas because this router is used to connect to Area 0. What commands would be used to identify the problem? In examining the diagram and configuration, what problems can you see?

Router A is configured incorrectly. The command that would show the problem would be either show ip route , show ip protocols , or show ip ospf database . The lack of LSA traffic would indicate a configuration problem. When examining the configuration, you would see that the mask on the configuration of the network command for Area 0 is wrong. The mask for 140.100.32.0 should be 0.0.0.15. Using the mask of 0.0.0.0 will place only interfaces with the IP address 140.100.32.0, which is a subnet address not an interface address. Therefore, there will be no communication of OSPF LSAs between the areas.

3. Issue the commands that would be used to correct the configuration problems that you see in the example configuration for Routers A and B.

The commands that would solve these problems are as follows:

4. When you issue the show ip ospf interface command, you notice that there is a discrepancy in the timers on the link between Routers A and B. The transmit timer on Router A is set to 5, and the retransmit timer is set to 1. What problems would this cause? What command would be used to change the timers, and what are the default settings?

The default setting for the transmit timer is set to 1 second, and the retransmit timer is set to 5 seconds. The transmit timer determines the estimated number of seconds that it takes to send a LSA to a neighbor. The retransmit timer states the number of seconds to wait for an acknowledgment before retransmitting an LSA.

If the transmit timer is not smaller than the retransmit timer, the interface retransmits in the belief that the other side did not receive the LSA. This leads to excess traffic, confusion in the topology database, and the possibility of flapping links. To correct the settings, issue the following subinterface commands:

5. There is an ISDN link into the Internet from Router B. The network manager has suggested that this link is the cause of some performance problems on the router. You have noticed that the interface is included in the OSPF network command. What might be the cause of the problem, and how could it be fixed?

If the ISDN interface is configured for dial-on-demand routing (DDR) and is also included in OSPF network commands, you might find that the link that the DDR process establishes will cause the routing updates to be propagated throughout the network. This causes additional CPU utilization on the routers and flooding of packets throughout the network. The solution is to ensure that the interface is not included in the network command to the OSPF process. A more important problem is that Router B is in a stub area and will not track external routes. Router B cannot connect to the Internet as an ASBR because it will not propagate the Type 5 LSAs. The BRI interface cannot partake in the OSPF network. Therefore, the network will not be slow; it will be down.

Scenario 9-3 Answers
1. Explain the purpose of the virtual link in Figure 9-11.

In this example, Area 1 does not have a direct physical connection into Area 0. A virtual link must be configured between Router A and Router B. Area 2 is to be used as a transit area, and Router B is the entry point into Area 0. This way, Router A and Area 1 will have a logical connection to the backbone.

2. Is the configuration of the OSPF network shown in Figure 9-12 a valid configuration?

Yes, the configuration is valid.

3. Why would a company implement this design?

OSPF allows for linking discontinuous parts of the backbone using a virtual link. In some cases, different Area 0s need to be linked together. This can occur, for example, if a company is trying to merge two separate OSPF networks into one network with a common Area 0. In other instances, virtual links are added for redundancy in case some router failure causes the backbone to be split in two. Whatever the reason may be, a virtual link can be configured between separate ABRs that touch Area 0 from each side and that have a common area between them.

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