Configuring OSPF Across Multiple Areas | Kickoff

Configuring OSPF Across Multiple Areas

12 Mar

This chapter covers the following topics, which you need to understand to pass the CCNP/CCDP/CCIP BSCI exam:
■ Required configuration commands for a multiarea OSPF network
■ Optional configuration commands for a multiarea OSPF network
■ A working configuration of OSPF on a multiarea network
■ Verifying the configuration of OSPF in a multiarea network
■ Troubleshooting a multiarea OSPF network

Configuring OSPF Across Multiple Areas
The topics in this chapter describe the configuration of the routing protocol OSPF across multiple areas. This chapter assumes your knowledge of Chapter 6, “Using OSPF in a Single Area,” and Chapter 7, “Configuring OSPF in a Single Area,” which dealt with OSPF configuration in a single area, and Chapter 8, “Using OSPF Across Multiple Areas,” which introduced you to using OSPF in multiple areas. This chapter builds on your understanding and explains the configuration of OSPF over a large multiarea network. You will learn how to implement and manage an OSPF network in a real environment, one of multiple areas.

Some of the commands in this chapter are the same as those needed in the configuration of OSPF in a single area, in particular the required commands and the verification and troubleshooting commands. This chapter briefly reconsiders the OSPF commands required for configuration in order to put the multiarea commands in context. This chapter concentrates on the configuration of a multiarea environment.

“Do I Know This Already?” Quiz
The purpose of the “Do I Know This Already?” quiz is to help you decide what parts of this chapter to use. If you already intend to read the entire chapter, you do not necessarily need to answer these questions now.

The 13-question quiz, derived from the major sections in the “Foundation Topics” portion of the chapter, helps you to determine how to spend your limited study time.

Table 9-1 outlines the major topics discussed in this chapter and the “Do I Know This Already?” quiz questions that correspond to those topics.

292 Chapter 9: Configuring OSPF Across Multiple Areas
Table 9-1 “Do I Know This Already?” Foundation Topics Section-to-Question Mapping

OSPF Across Multiple=

CAUTION The goal of self-assessment is to gauge your mastery of the topics in this chapter. If you do not know the answer to a question or are only partially sure of the answer, you should mark this question wrong for purposes of the self-assessment. Giving yourself credit for an answer you correctly guess skews your self-assessment results and might provide you with a false sense of security.

1. Which of the following is the correct command for configuring OSPF?
a. Router(config)# router ospf process-number
b. Router(config)#router ospf router-ID
c. Router(config)#router ospf autonomous-system-number
d. Router(config-if)#router ospf process-id

2. Which of the following commands is the correct command for placing the subnet 172.16.20.128 in Area 1, and all other subnets within the classful address in Area 0?
a. network 172.16.20.128 0.0.0.0 area 1
network 0.0.0.0 255.255.255.255 area 0
b. network 172.16.20.128 0.0.0.255 area 1
network 172.16.0.0 0.0.255.255 area 0
c. network 172.16.20.128 0.0.0.0 area 1
network 172.16.20.0 255.255.255.255 area 0
d. network 172.16.20.128 0.0.0.7 area 1
network 172.16.20.0 255.255.255.255 area 0

3. Which of the following statements are true about placing subnets into different areas?
a. The network command used to assign interfaces and subnets to different areas can be configured on any OSPF router.
b. The network command is used at the interface level to assign interfaces and their subnets into different areas.
c. The network command uses the subnet mask to identify the portion of the address to consider in assigning areas.
d. The network command uses a wildcard mask to identify the portion of the address to consider in assigning areas.

4. Which of the following statements are true of the command summary-address 172.16.20.0
255.255.255.0 ?
a. This command would be configured on an ASBR.
b. This command is used to summarize routes between areas.
c. This command is used to summarize addresses advertised to the outside world.
d. This command is configured on ABR routers.

5. Which command would be used on an ABR to configure a cost of a default route propagated into a stub or NSSA area?
a. default-cost 30
b. area 1 default-cost 30
c. ip ospf default cost 30
d. ip ospf area 1 default-cost 30

6. What is the purpose of the command area 3 stub no-summary ?
a. To create Area 3 as a totally stubby area
b. To create Area 3 as a stub area
c. To create Area 3 as an NSSA area
d. To restrict summaries passing between areas

7. Which of the following commands shows the correct syntax for defining a virtual link?
a. area 1 virtual-link 10.10.10.10
b. ip ospf area 1 virtual-link 10.10.10.10
c. ip ospf virtual-link area 1
d. area 1 virtual-link 10.10.10.0 255.255.255.0 172.16.20.131 255.255.255.0

8. Which command is used to show the OSPF ABRs and ASBRs for which the internal router has entries in its routing table?
a. show ip ospf external routers
b. show ip ospf route
c. show ip ospf border database
d. show ip ospf border-routers

9. Which of the following is the correct command to show virtual links?
a. show virtual-links
b. show ip ospf virtual-links
c. show ip ospf links
d. show ip ospf neighbors

10. Which of the following are shown in the show ip ospf database command? a. Router ID number
b. Hello timer intervals
c. Advertising router ID
d. Number of interfaces detected for router

11. If a router configured for OSPF routing is not seeing an OSPF neighbor on an attached network, which of the following actions should be taken?
a. Ensure that the OSPF autonomous system number is configured correctly on both ends
b. Make sure that both routers are configured with the same IP mask, Interface Hello timer,
OSPF Hello interval, and OSPF dead interval
c. Make sure that both neighbors are part of the same area and area type
d. Ensure the virtual links are configured in the same subnet

12. Which of the following events are shown the debug ip ospf events command?
a. Adjacencies
b. Designated router selection
c. Virtual links
d. SPF calculations

13. Which of the following commands is useful for analyzing the messages traveling between the local and remote hosts?
a. show ip ospf traffic
b. debug ip ospf data
c. debug ip packet
d. show ip ospf retransmission-list

The answers to the “Do I Know This Already?” quiz are found in Appendix A, “Answers to Chapter ‘Do I Know This Already?’ Quizzes and Q&A Sections.” The suggested choices for your next step are as follows:

■ 6 or less overall score —Read the entire chapter. This includes the “Foundation Topics” and “Foundation Summary” sections, the “Q&A” section, and the “Scenarios” at the end of the chapter.
■ 7–9 overall score —Begin with the “Foundation Summary” section, and then go to the “Q&A” section and the “Scenarios” at the end of the chapter. If you have trouble with these exercises, read the appropriate sections in “Foundation Topics.”
■ 10 or more overall score —If you want more review on these topics, skip to the “Foundation Summary” section, and then go to the “Q&A” section and the “Scenarios” at the end of the chapter. Otherwise, move to the next chapter.

Foundation Topics
Required Configuration Commands for a Multiarea OSPF Network

As you saw in Chapter 7, it is necessary to start the routing process on the router in order to define how it is to operate. As part of starting the process, you must tell the process how to run. Many OSPF commands are optional, as described in the section “Optional Configuration Commands for a Multiarea OSPF Network,” but the following need to be defined at the startup of the process:

■ Participating router interfaces —The router might not want to have all its interfaces send or receive OSPF routing updates. A classic example is a dialup line to a remote office. If there is only one subnet at the remote office, it would be more efficient to use default and static route commands, because any updates would dial the line.

■ Identification of the area —The router will define which area it is in on a per-interface basis.
■ A router ID —This allows the router to be uniquely identified by a single address. The ID of the router advertising a link can be used to determine the next logical hop, for example, if that link is used in the path selection to a remote network.

The following commands, which you will learn more about in the next sections, are necessary to configure a multiarea OSPF network:
■ The OSPF router command
■ The OSPF network command

Enabling the OSPF Routing Protocol
When configuring the router for the first time, there is no IP routing protocol running on the Cisco router (unless the SETUP script is used). This is not true of other protocols, however (refer to Chapter 7).

To configure OSPF as the routing protocol, use the following command:

Router(config)#router ospf process-number

Recall that process-number is a number local to the router. It is possible to have more than one process running on a router, although this is an unusual and expensive configuration in terms of router resources. The process number does not have to be the same on every router in the area or the autonomous system. In the interest of sanity, however, many administrators assign the same number to the routers.

NOTE A common error in configuration is to confuse the process ID with the router ID or the area ID. These are not related in any way. The process ID is simply a mechanism to allow more than one process to be configured on a router. The router ID is the mechanism by which a router is identified within the OSPF domain, and the area ID is a mechanism of grouping routers that share full knowledge of OSPF-derived routes within the OSPF area.

Enabling the network Command
The network command was explained in Chapter 7 in terms of identifying the interfaces that participated in the OSPF routing process. In this chapter, the network command is used to identify not only the interfaces that are sending and receiving OSPF updates, but also the area in which they reside. Defining the areas with the network command is used on an area border router (ABR).

The following is the syntax for the OSPF network command:
Router(config-router)#network network-number wildcard-mask aarreeaa area-number

NOTE The area requested in the preceding syntax is the area in which the interface or interfaces configured with the network address reside.

You must take care in the use of the wildcard mask. In a single-area configuration, all the interfaces are in the same area. The network commands just identify the network numbers in use. Therefore, they can be configured to the classful network address provided by the IANA, as they are in IGRP and RIP. The only reason to be more specific would be to exclude some interfaces from the OSPF domain.

Example 9-1 shows two interfaces, each with a subnet of the same major network where the interfaces are placed into different areas. The network number has been subnetted into the last octet so that you can truly see the power and granularity of the wildcard mask at work. Figure 9-1 illustrates this configuration.

Example 9-1 The network Command for Router A

network Command for Router

298 Chapter 9: Configuring OSPF Across Multiple Areas
Figure 9-1 The network Command

Multiple Areas

Optional Configuration Commands for a Multiarea OSPF Network
The word optional is used here to mean not absolutely necessary, implying that OSPF will run without the optional configuration commands. This does not mean that OSPF runs well or efficiently without them, however. A few of the OSPF commands, optional or not, are necessary in the configuration of an efficient multiarea OSPF network. The following list shows those optional OSPF commands that are important to the maintenance of an efficient network:

■ The area range command configured on an ABR
■ The summary-address command for use on an autonomous system border router (ASBR)
■ The area area-id stub command to define a stub area
■ The area area-id stub no-summary command to define a totally stubby area

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■ The area default-cost command for determining the cost of default routes that enter the area
■ The area virtual-link commands used to create a virtual link

The area range Command
The area range command is configured on an ABR because it dictates the networks that will be advertised out of the area.

Use the area router configuration command with the range keyword to consolidate and summarize routes at an area boundary. This reduces the size of the databases, which is particularly useful in the backbone area because it is the transit area for all the other areas. Use the no form of this command to disable this function for the specified area:

Router(config-router)#area area-id rraannggee address mask
Router(config-router)#no area area-id rraannggee address mask

In the preceding syntax, area-id is the identifier (ID) of the area about which routes are to be summarized. It can be specified as either a decimal value or an IP address. Here, address is the IP address, and mask is the IP mask.

Example 9-2 shows the configuration required to summarize the following five individual subnets (which can address six hosts each) into one subnet. This summarized subnet can then be propagated across the OSPF network, saving both bandwidth and CPU:
■ 172.16.20.128/29
■ 172.16.20.144/29
■ 172.16.20.160/29
■ 172.16.20.176/29
These subnets are summarized into one subnet:
■ 172.16.20.128/26

This one subnet will then be propagated into Area 1 (see Figure 9-2).
Example 9-2 The OSPF area range Command for an ABR
propagated into Area

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Figure 9-2 The OSPF area range Command for an ABR

Command for an ABR

NOTE The area ID requested is the area from which the subnets originated. It is not the destination area.

The summary-address Command
The summary-address command is used on the ASBR to summarize the networks to be advertised to the outside world. Routes received into OSPF via redistribution from another routing protocol are seen as external routes. The summary-address command allows the summarization of routes to be redistributed into OSPF from other routing protocols.

The syntax for the OSPF summary-address command for an ASBR is as follows:

Router(config-router)#summary–address address mask [not-advertise][tag tag]

In the preceding syntax, address is the summary address designated for a range of addresses, and mask is the IP subnet mask used for the summary route.

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The design and implementation of the addressing scheme are crucial to the success of the OSPF network and cannot be stressed too strongly. Refer to Chapter 2, “IP Addressing,” for information about summarization and Chapter 3, “Designing IP Networks,” for details on IP addressing.

Example 9-3 shows the summarization of the network address 172.16.20.0, received from the ISP and propagated throughout the autonomous system. Figure 9-3 illustrates this example.

Example 9-3 The OSPF summary-address Command for an ASBR

summary-address

Figure 9-3 The OSPF summary-address Command for an ASBR

summary-address

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The area area-id stub Command
After designing the addressing scheme for the network, it should be clear which areas, if any, are suitable candidates for configuration as stub, totally stubby, or not so stubby areas.

The syntax for the OSPF router command for a stub area is as follows:

Router(config-router)#area area-id stub

Example 9-4 shows the creation of a stub area. Note that both the ABR and the internal router share the stub area configuration. Figure 9-4 illustrates this example.
Example 9-4 The Configuration of a Stub Area

Configuration of a Stub Area

NOTE All OSPF routers inside a stub area must be configured as stub routers. Whenever an area is configured as a stub, all interfaces that belong to that area will start exchanging Hello packets with a flag that indicates that the interface is a stub. The flag is a bit in the Hello packet called the E bit. The E bit is set to 0. All routers that have a common segment must agree on this flag. If they do not, they will not become neighbors, and routing will not take effect.

The area area-id stub no-summary Command
The syntax for the OSPF command for a totally stubby area is as follows:

Router(config-router)#area area-id stub no-summary

This addition of the no-summary parameter informs the ABR not to send summary updates from other areas into the area. This command needs to be configured only on the ABR because it is the only router with this responsibility. This command is configurable only on a Cisco router because it is a proprietary feature. All the other routers are configured as stub-area internal routers.

Example 9-5 shows the configuration of a totally stubby area. Figure 9-5 illustrates this example.

Example 9-5 The Configuration of a Totally Stubby Area
Configuration of a Totally

As a totally stubby area, no summary or external routes are propagated by the ABR into the area. Toreach networks and hosts outside their area, a workstation must send to a default route, which the ABR advertises into the area.

The area default-cost Command
To define the cost to the default route, the following command is used. If the cost is not specified, the path will be calculated as the internal area cost plus 1:

Router(config-router)#area area-id default-cost cost

The ABR attached to the stub area automatically generates and advertises a default route with a destination of 0.0.0.0 into the stub area.

304 Chapter 9: Configuring OSPF Across Multiple Areas
Figure 9-5 The Configuration of a Totally Stubby Area

Configuration

Example 9-6 The OSPF Command for the Default Route Propagated into the Area

OSPF Command

Optional Configuration Commands for a Multiarea OSPF Network 305

Example 9-6 shows how the default cost can be set in the configuration. Setting a cost on the default route is useful when the stub area has more than one ABR. The default cost allows the ABR used to exit the area. Figure 9-6 illustrates this example.

Figure 9-6 The OSPF Command for the Default Route Propagated into the Area

illustrates

You need to configure the area default-cost command only on the ABR because it is the only router with this responsibility. However, Example 9-6 shows the configuration on both routers to illustrate the choice. The second ABR, Router E, will only be used if Router A fails. If there were no configuration on Router A, it would still be used by all internal routers as the ABR because the default cost is 1.

The area virtual-link Command
When it is not possible to connect an area to Area 0 directly, one solution is to create a tunnel called a virtual link. This is remarkably easy to configure. As with many things in OSPF, of course, this ease of configuration belies the complexity of the technology being used. Many things can go wrong. The most common problem is in the address of the other end of the virtual link. The command area virtual-link is configured between ABRs that share a common area; at least one of the ABRs must be in Area 0. The command, issued at both ABRs, states the transit area and the router ID of the remote destination ABR. This creates essentially a tunnel through the transit area,
which, although it might involve many routers to forward the traffic, appears to the remote ABRs as next hops.

NOTE The area area-id virtual-link command might be included in the BSCI exam and, for that reason, is worth mentioning. In practice, virtual links are a design nightmare and are best avoided. They are useful when mending a network on a temporary basis while awaiting a moment’s peace to rectify the design of the network.

The syntax to configure a virtual link is as follows:

Router(config-router)#area area-id virtual-link router-id

Here, area-id is the ID assigned to the transit area for the virtual link. In addition, router-id is the
router ID of the virtual link neighbor.

Example 9-7 shows the setting of the loopback interfaces that provide the router ID. It then shows the configuration of the virtual link through the network. Figure 9-7 illustrates this example.

Example 9-7 Configuring a Virtual Link

Virtual Link

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Figure 9-7 Configuring a Virtual Link

Configuring a Virtual Link

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