Chapter 7. Using OSPF Across Multiple Areas

This chapter covers the following topics:

The Purpose of Multi-area OSPF—Explains OSPF multiple-area networks and the single area network issues they solve.
The Features of Multi-area OSPF—Describes router types, link-state advertisements, and the different types of areas.
The Operation of Multi-area OSPF—Explains LSA propagation, OSPF path selection between areas, and how OSPF calculates the costs of paths to other areas.
Design Considerations in Multi-area OSPF—Describes capacity planning, summarization, and the use of virtual links in multi-area OSPF.
Required Configuration Commands for a Multi-area OSPF Network—Describes configuring OSPF as the routing protocol and explains the network command in the context of OSPF.
Optional Configuration Commands for a Multi-area OSPF Network—Describes commands that ensure the multi-area OSPF network runs efficiently.
Working Configuration of Multi-area OSPF—Combines the concepts and commands described in the chapter to show an entire working configuration.
Verifying the Configuration of OSPF in a Multi-area Network—Covers the commands used to verify the configuration and monitor the network.
Troubleshooting a Multi-area OSPF Network—Covers the log-adjacency-changes command, the most useful debug commands, and common problems that occur in forming adjacencies.

This chapter discusses using OSPF in large networks—networks large enough to be broken up into multiple smaller areas for routing purposes. In order to design a multi-area OSPF network, you need a comprehensive grasp of the features of multi-area OSPF, the operation of OSPF across multiple areas, and, of course, the design considerations of such a network. This chapter discusses each of these topics.

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

Table 7-1 outlines the major topics discussed in this chapter and the "Do I Know This Already?" quiz questions that correspond to those topics.

Table 7-1. "Do I Know This Already?" Foundation Topics Section-to-Question Mapping
Foundation Topics Section	Questions Covered in This Section	Score
The Purpose of Multi-area OSPF	1
The Features of Multi-area OSPF	2–6
The Operation of Multi-area OSPF	7
Design Considerations in Multi-area OSPF	8–11
Required Configuration Commands for a Multi-area OSPF Network	12–14
Optional Configuration Commands for a Multi-area OSPF Network	15–18
Working Configuration of OSPF in a Multi-area Network	19
Verifying the Configuration of OSPF in a Multi-area Network	20–22
Troubleshooting a Multi-area OSPF Network	23
Total Score

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.	What is the command to view CPU utilization? show memory free show cpu utilization show processes cpu sorted show processor
2.	What is an internal router? An OSPF process running on a multilayer switch. The internal router is responsible for managing multiple OSPF processes. All interfaces on this router are within the same area. A router running OSPF with no external processes.
3.	What is the purpose of an ABR? A router responsible for connecting to outside the autonomous system A router responsible for connecting two or more areas A logical group of nodes forming a backbone that connects other areas A group of routers running OSPF with no external links

4.	What do the initials ASBR represent? Autonomous System Border Router Authority Subnet Boundary Router Automatic Summarization Boundary Router Autonomous System Boundary Router
5.	How is routing information generated within an area propagated throughout the area? Type 3 and Type 4 LSAs In the summary LSA sent out every 30 minutes In the Hello packet between neighbors Type 1 and Type 2 LSAs
6.	Which of the following OSPF characteristics affect how the routing table is created? Whether there are multiple areas in the domain Whether MD-5 security has been configured The role of the router in the area in which the router is located Whether there are communications outside the autonomous system Network link LSA
7.	Which two LSA types are propagated within an area? Router Type 3 Type 4 Type 5 Network
8.	Why does the frequency of the SPF algorithm increase with the size of the area? Each router will have more neighbors, and synchronizing the databases takes priority. The topology databases will subdivide after a certain size, requiring multiple SPF calculations. As the area size increases, each recalculation will also take longer, which might result in errors, requiring the algorithm to rerun. The larger the network, the greater the probability of a network change and, thus, a recalculation of the entire area.

9.	Multiple areas are one of the main features of OSPF. Which of the following statements explains why this feature is such an important enhancement to earlier routing protocols? It is easier to implement security. All computation is kept within the area, with minimum communication between the areas, allowing the network to scale to larger sizes. The network domain, when divided into areas, allows for the use of both IANA classful addressing and private addressing. The use of multiple areas allows for the use of prioritization.
10.	Which is the best design for OSPF? Hierarchical with summarization Tiered Flat with summarization Elliptical
11.	Which of the following must be observed when creating a virtual link? Both routers must share a common area. Both routers must share the same subnet address. One of the routers must be connected to Area 0. Both routers must share the same process ID.
12.	What command is used to start an OSPF process? Router(config)# router ospf process-id Router(config)# router ospf router-ID Router(config)# router ospf autonomous-system-number Router(config-if)# router ospf process-id

13.	Which of the following commands are the correct commands, in conjunction, for placing an interface with IP address 172.16.20.128 in area 1 and the interfaces associated with the 172.16.0.0 class B network in Area 0? network 172.16.20.128 0.0.0.0 area 1 network 0.0.0.0 255.255.255.255 area 0 network 172.16.20.128 0.0.0.255 area 1 network 172.16.0.0 0.0.255.255 area 0 network 172.16.20.128 0.0.0.0 area 1 network 172.16.20.0 255.255.255.255 area 0 network 172.16.20.128 0.0.0.7 area 1 network 172.16.20.0 255.255.255.255 area 0
14.	Which of the following statements about placing subnets into different areas are true? The network command used to assign interfaces and subnets to different areas can be configured on any OSPF router. The network command is used at the interface level to assign interfaces and their subnets into different areas. The network command uses the subnet mask to identify the portion of the address to consider in assigning areas. The network command uses a wildcard mask to identify the portion of the address to consider in assigning areas.
15.	Which of the following statements are true of the command summary-address 172.16.20.0 255.255.255.0? This command would be configured on an ASBR. This command is used to summarize routes between areas. This command is used to summarize addresses advertised to the outside world. This command is configured on ABR routers.
16.	Which command would be used on an ABR to configure the cost of a default route propagated into a stub or NSSA area? default-cost 30 area 1 default-cost 30 ip ospf default cost 30 ip ospf area 1 default-cost 30

17.	What is the purpose of the command area 3 stub no-summary? To create area 3 as a totally stubby area To create area 3 as a stub area To create area 3 as an NSSA area To filter summaries passing between areas
18.	Which of the following commands shows the correct syntax for defining a virtual link? area 1 virtual-link 10.10.10.10 ip ospf area 1 virtual-link 10.10.10.10 ip ospf virtual-link area 1 area 1 virtual-link 10.10.10.0 255.255.255.0 172.16.20.131 255.255.255.0
19.	In the following configuration, which areas will have default routes automatically propagated to the routers in the respective areas? RouterA(config)# router ospf 100 RouterA(config-router)# network 140.100.17.128 0.0.0.15 area 3 RouterA(config-router)# network 140.100.17.192 0.0.0.15 area 2 RouterA(config-router)# network 140.100.32.0 0.0.0.255 area 0 RouterA(config-router)# area 2 stub RouterA(config-router)# area 3 stub no-summary RouterA(config-router)# area 3 default-cost 15 Area 2 Area 3 Neither area Both areas
20.	Which command is used to show the OSPF ABRs and ASBRs for which the internal router has entries in its routing table? show ip ospf external routers show ip ospf route show ip ospf border database show ip ospf border-routers

21.	Which of the following is the correct command to show virtual links? show virtual-links show ip ospf virtual-links show ip ospf links show ip ospf neighbors
22.	Which of the following are shown in the show ip ospf database command? Router ID number Hello timer intervals Advertising router ID Number of interfaces detected for the router
23.	Which of the following events are shown in the debug ip ospf events command? Adjacencies Designated router selection Virtual links SPF calculations

The answers to this 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:

13 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.
16–19 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."
20–23 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.