Part IV: IS-IS

This chapter covers the following topics:

  • Introduction to Integrated IS-IS—Describes the history of IS-IS and explains why IS-IS continues to be of interest to networkers.

  • Comparing OSPF and IS-IS—Describes the similarities and differences between OSPF and IS-IS.

  • ISO Addressing for Integrated IS-IS—Describes the addressing structure for ISO.

  • IS-IS Adjacency—Describes how neighborship is formed.

  • Integrated IS-IS Operation—Discusses the operation of IS-IS.

  • Integrated IS-IS Design Considerations—Describes design issues, such as where to summarize and how to plan an IS-IS infrastructure.

The topics in this chapter detail using Intermediate System-to-Intermediate System (IS-IS) as an IP routing protocol. This chapter assumes knowledge of routing protocols and, in particular, link-state routing protocols. This chapter introduces Integrated IS-IS by explaining the protocol's terminology and fundamental concepts.

Because Integrated IS-IS is similar to the Open Shortest Path First (OSPF) protocol, you should read the chapters on OSPF and reinforce your knowledge of the fundamentals of link-state protocols. Differences between Integrated IS-IS and OSPF are clearly outlined within this chapter.

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 22-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 9-1 outlines the major topics discussed in this chapter and the "Do I Know This Already?" quiz questions that correspond to those topics.

Table 9-1. "Do I Know This Already?" Foundation Topics Section-to-Question Mapping
Foundation Topics SectionQuestions Covered in This SectionScore
Introduction to Integrated IS-IS1–4 
Comparing IS-IS and OSPF5–10 
ISO Addressing for Integrated IS-IS11–13 
IS-IS Adjacency14–17 
Integrated IS-IS Operation18–19 
Integrated IS-IS Design Considerations20–22 
Total Score  


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 packets are used by IS-IS?
  1. Hellos

  2. LSPs

  3. TVLs

  4. SNPs

2.Which of the following are valid hello packet types for IS-IS?
  1. Hello Level 1 LAN

  2. Hello Level 2 LAN

  3. Hello Level 1-2 LAN

  4. Hello Point-to-Point

3.IS-IS uses the Hello protocol to create and maintain adjacencies and neighbor relations. Which of the following are Hello packets used in IS-IS?
  1. A generic Hello

  2. Point-to-point Level 1, point-to-point Level 2, LAN Level 1, LAN Level 2

  3. Point-to-point, LAN Level 1, LAN Level 2

  4. Point-to-point Level 1, point-to-point Level 2, LAN

4.What does TLV stand for?
  1. Total/Length/Verification

  2. Type/Length/Value

  3. Topology/Long/Vector

  4. Temp/Length/Vector

5.What does the acronym CSNP represent?
  1. Current System Node Packet

  2. Complete Sequence Number Protocol

  3. Code Sequence Number Protocol

  4. Complete sequence number packet

6.Which description best fits a Level 1 router?
  1. An interarea router

  2. An HODSP router

  3. An internal router

  4. An intra-area router

7.A Level 2 router is similar to which type of OSPF router?
  1. ASBR

  2. ABR

  3. Stub

  4. Backbone internal router

8.Where are IS-IS areas defined?
  1. On the link

  2. On the interface

  3. At the process

  4. A separate process

9.Which of the following statements is true about Level 2 routers?
  1. Level 2 routers can send updates between routing areas.

  2. Level 2 routers must be placed contiguously.

  3. Level 2 routers are similar to OSPF stub routers.

  4. Level 2 routers must be in the same area.

10.Which of the following do IS-IS and OSPF have in common?
  1. Dijkstra algorithm

  2. Classless routing protocol

  3. Link-state routing protocol

  4. Summarization at the area boundary

11.ISO 10589 defines the NET as having three fields. What are these fields?
  1. IDP, DSP, SEL

  2. Area, System ID, NSEL

  3. IDP, DSP, System ID


12.Which of the following is a valid NET address?
  1. 47.0005.aa00.0301.16cd.00

  2. 47.0005.aa00.0301.16cd.01

  3. 47.0005.aa00.0301.16cd.ff

  4. 47.0005.aa00.19g6.3309.00

13.The NET system ID for a Level 2 router must be unique across which range?
  1. Interface

  2. Level 1 Area

  3. Backbone

  4. Global

14.For an IS-IS adjacency to be formed and maintained, both interfaces must agree on which of the following?
  1. If Level 1, both must be in the same area.

  2. The system ID must be unique.

  3. Both must be configured at the same level of routing (1 or 2).

  4. Hello timers.

15.How does the pseudonode represent all the routers on the LAN?
  1. Each router on the multiaccess LAN simulates an interface on the pseudonode.

  2. The pseudonode represents the physical link to which the routers are connected.

  3. The pseudonode is the virtual link between two discontiguous areas.

  4. Each link on every router on the LAN is summarized to form a pseudonode for the LAN.

16.A point-to-point link uses which packets to create and maintain neighbor tables and link-state databases?
  1. PSNP

  2. Hellos

  3. CSNP

  4. LSP

17.What is used to elect the DIS automatically?
  1. Highest SNPA address.

  2. Priority defined at the interface.

  3. The DIS must be manually configured.

  4. The highest loopback address.

18.What triggers an LSP update to be flooded to neighbors?
  1. When an adjacency goes up or down

  2. When a change occurs in the state of the interface

  3. When a change occurs in the routing table

  4. When an LSP is received

19.What is the default cost applied to the outgoing interface of a Cisco router configured to run IS-IS?
  1. 15

  2. 10

  3. Inverse of the bandwidth.

  4. There is no default. It must be configured.

20.Which of the following are IS-IS rules for summarization?
  1. All Level 1 routers can summarize routes.

  2. All Level 2 routers can summarize at the area boundary.

  3. If a Level 1-2 router is summarizing routes sent to a Level 2 router, all Level 1-2 routers must summarize in the same way.

  4. All the answers provided are correct.

21.What is the advantage of designing a totally flat network running Level 1-2 routing on every router?
  1. Easy migration to multiple areas.

  2. Easy system administration.

  3. It requires fewer resources from either the network or the router.

  4. Summarization can be configured within the area.

22.What would result if the only Level 1-2 router in an area failed?
  1. Another router would be elected as the Level 1-2 router.

  2. The autonomous system would not be able to communicate with the outside world.

  3. The area would be completely cut off from the rest of the network.

  4. No summary routes would be sent into the backbone area.

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:

  • 14 or less overall score—Read the entire chapter. This includes the "Foundation Topics," "Foundation Summary," and "Q&A" sections.

  • 15–18 overall score—Begin with the "Foundation Summary" section, and then go to the "Q&A" section at the end of the chapter. If you have trouble with these questions, read the appropriate sections in "Foundation Topics."

  • 19 or more overall score—If you want more review on these topics, skip to the "Foundation Summary" section and go to the "Q&A" section at the end of the chapter. Otherwise, move to the next chapter.