Chapter 5. Understanding Simple Single-Area OSPF

This chapter covers the following topics:

Understanding OSPF Fundamentals—This section reviews fundamental concepts related to OSPF operation.
Configuring OSPF in a Single Area—This section describes required and optional commands for configuring OSPF in a single area.
Checking the Configuration of Single-Area OSPF—This section describes commands used to troubleshoot the OSPF configuration.

This chapter explores the routing protocol Open Shortest Path First (OSPF). You should already be familiar with the basic concepts of link-state routing protocols and single-area OSPF. This chapter briefly reviews single-area OSPF and then introduces multiple-area OSPF. The chapter concludes with techniques for configuring, verifying, and troubleshooting OSPF.

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 14-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 5-1 outlines the major topics discussed in this chapter and the "Do I Know This Already?" quiz questions that correspond to those topics.

Table 5-1. "Do I Know This Already?" Foundation Topics Section-to-Question Mapping
Foundation Topics Section	Questions Covered in This Section	Score
Understanding OSPF Fundamentals	1–10
Configuring OSPF in a Single Area	11–12
Checking the Configuration of Single-Area OSPF	13–14
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 a neighbor on a local LAN segment in OSPF? A neighbor is a router in the same area. A neighbor is a router in the same classful network. A neighbor is a router on a multiaccess link, with an adjacency with a DR. A neighbor is another router with the same network address.
2.	What is an adjacency in OSPF? An adjacency is when another router has received an LSA from another area. The areas are adjacent. An adjacency is the state that two neighbors can achieve after they have synchronized their OSPF databases. An adjacent router is one that has received a hello packet from a neighbor. Routers connected across a WAN but not directly connected, for example Frame Relay, are considered adjacent to each other.
3.	What is a designated router? The router responsible for maintaining the SPF tree for a totally stubby area The router responsible for summarizing routes to other areas A router responsible for making adjacencies with all routers on a multiaccess link and maintaining those adjacencies The router responsible for forwarding all the traffic across the global Internet
4.	By default, how often does OSPF send out hello packets on a broadcast multiaccess link? By default, OSPF sends out hello packets every 30 seconds on a broadcast network. By default, OSPF sends out hello packets every 40 seconds on a broadcast network. By default, OSPF sends out hello packets every 3.3 seconds on a broadcast network. By default, OSPF sends out hello packets every 10 seconds on a broadcast network.

5.	When a router sends an LSA on a multiaccess link, to what is it sent? The DR, which updates the BDR every hello interval. The DR and the BDR. All routers on the link; all routers maintain adjacencies, but only the DR updates the rest of the network. The DR, which updates the BDR every 3.3 minutes.
6.	What does it mean when an interface shows that it is in the init state? That an interface is coming online, determining the IP address and OSPF parameters That a router sees a hello packet from a neighbor but the packet does not contain its own router ID That this is a point-to-multipoint interface and is waiting to connect to the WAN cloud Seen only on broadcast links, it shows that the election of the DR is in progress
7.	If the network is stable and sees no changes, how often will it send LSAs? Why are these updates sent out periodically? Every 30 minutes by default. This is to ensure the integrity of the topological databases. Every 30 seconds by default. This is to ensure that the network is fully connected. Never; there is no need if the network is stable. Whenever an LSA is received; this means there is a problem on the network that needs to be flooded through the network.
8.	In learning a new route, what will an internal OSPF router do if a received LSA is not found in the topological database? The LSA is flooded immediately out of all the OSPF interfaces, except the interface from which the LSA was received. The LSA is dropped and a message is sent to the transmitting router. The LSA is placed in the topological database and an acknowledgement is sent to the transmitting router. The sequence numbers are checked, and if the LSA is valid, it is entered into the topology database.

9.	What would the default cost of a 1-Mbps link be? 1 10 100 1000
10.	If a router has an OSPF priority set to 0, what does this indicate? A router with the OSPF priority set to 0 is one that can participate in the election of a DR. It has the highest priority. A router with the OSPF priority set to 0 is one that will switch OSPF packets before it does anything else. A router with the OSPF priority set to 0 is one that cannot participate in the election of a DR. It can become neither a DR nor a BDR. A router with the OSPF priority set to 0 is one that cannot participate in the election of a DR, but it can become a BDR.
11.	Which of the following IP addresses are subsumed by this command: Router(config-router)# network 10.1.32.0 0.0.31.255 area 0 10.1.32.255 10.1.34.0 10.1.64.0 10.1.64.255
12.	What is the default OSPF cost for a T1 link? 1 10 32 64
13.	On a LAN segment, what is the maximum number of neighbors that can be designated as DROTHER in the show ip ospf neighbor output? 1 8 32 None of these options is correct

14.	Which command would include output such as "SPF algorithm executed 10 times?" show ip ospf show ip ospf interface show ip ospf neighbor show ip ospf database

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:

10 or less overall score—Read the entire chapter. This includes the "Foundation Topics," "Foundation Summary," "Q&A," and "Scenarios" sections at the end of the chapter.
11–12 overall score—Begin with the "Foundation Summary" section, and then go to the "Q&A" and "Scenarios" sections. If you have trouble with these exercises, read the appropriate sections in "Foundation Topics."
13–14 overall score—If you want more review on these topics, skip to the "Foundation Summary" section, and then go to the "Q&A" and "Scenarios" sections. Otherwise, move to the next chapter.