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 you 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 designed to challenge you intellectually: 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 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 5-1 Answers

1.Which parameter determines the router that will be selected as the DR in an OSPF network?
The router with the highest OSPF priority on a segment will become the DR for that segment. The default priority is 1. If multiple routers have the same priority, the router with the highest RID will be selected as the DR.
2.Could a router with a priority value of zero assume the role of a DR or BDR in the OSPF?
No. A priority value of zero indicates that an interface is not to be elected as a DR or BDR. The state of the interface with priority zero will be DROTHER.
3.How is the OSPF router ID determined on a Cisco router?
The OSPF router ID, if not manually configured, is the highest IP address on the box, or the highest loopback address, if one exists.
4.What is the role of the DR and the BDR in the OSPF network?
Instead of each router exchanging updates with every other router on the segment, every router will exchange the information with the DR and the BDR. The DR and the BDR will relay the information to everyone else. In mathematical terms, the number of adjacencies required for a full mesh is n(n–1)/2 and for a DR/BDR situation is 2n–2.