Part III: IS-IS
Chapter 10 Fundamentals of the Integrated IS-IS Protocol
Chapter 11 Integrated IS-IS Protocol Operation
Chapter 12 Configuring Integrated IS-IS
Part III covers the following Cisco BSCI exam topics:
■ Explain basic OSI terminology and network layer protocols used in OSI
■ Identify similarities and differences between Integrated IS-IS and OSPF
■ List the types of IS-IS routers and their role in IS-IS area design
■ Describe the hierarchical structure of IS-IS areas
■ Describe the concept of establishing adjacencies
■ Given an addressing scheme and other laboratory parameters, identify the steps to configure Cisco routers for proper Integrated IS-IS operation
■ Identify verification methods that ensure proper operation of Integrated IS-IS on Cisco routers
■ Interpret the output of various show and debug commands to determine the cause of route selection errors and configuration problems
This chapter covers the following topics, which you need to understand to pass the CCNP/CCDP/CCIP BSCI exam:
■ Introduction to Integrated IS-IS
■ OSPF and IS-IS compared
■ ISO addressing for Integrated IS-IS
■ Integrated IS-IS hierarchical structure
■ Basic principles of area routing
■ Integrated IS-IS networks and interfaces
■ Network layer protocols used in Integrated IS-IS
Fundamentals of the Integrated IS-IS Protocol
The topics in this chapter detail the routing protocol Integrated IS-IS. 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 the fundamentals of link-state protocols. Differences between Integrated IS-IS and OSPF are clearly outlined within this chapter.
“Do I Know This Already?” Quiz
The purpose of the “Do I Know This Already?” quiz is to help you to 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 21-question quiz, derived from the major sections in “Foundation Topics” portion of the chapter, helps you to determine how to spend your limited study time.
Table 10-1 outlines the major topics discussed in this chapter and the “Do I Know This Already?” quiz questions that correspond to those topics.
Table 10-1 “Do I Know This Already?” Foundation Topics Section-to-Question Mapping
336 Chapter 10: Fundamentals of the Integrated IS-IS Protocol
NOTE 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 does the initialism CLV stand for?
2. What do the initials CSNP represent?
a. Current System Node Packet
b. Complete Sequence Number Protocol
c. Code Sequence Number Protocol
d. Complete sequence number packet
3. What do the initials CLNS represent?
a. Connectionless Network Service
b. Connection Network Service
d. Complete Network Sequence
4. Which of the following do IS-IS and OSPF have in common?
a. Dijkstra algorithm
b. Classless routing protocol
c. Link-state routing protocol
d. Summarization at the area boundary
5. OSPF uses the term autonomous system. Which of the following is the equivalent IS-IS term?
b. Routeing Domain
c. Initial domain part
d. Authority format identiﬁer
6. OSPF assigns the network address to the interface. To what is the IS-IS address assigned?
a. To the CLNS interface
b. To the IANA network address
c. To the interface
d. To the IS-IS process
7. ISO 10589 deﬁnes the ISO address as having three ﬁelds. What are these ﬁelds?
a. IDP, DSP, SEL
b. Area, System ID, and SEL
c. IDP, DSP, System ID
d. AFI, IDP, HODSP
8. Which of the following is a valid NET address?
9. In the rules for ISO addressing, the System ID for a Level 2 router must be unique at which of the following levels?
a. The interface on the router
b. The area
c. The routing domain
10. A Level 1 router is described as which of the following?
a. An interarea router
b. An HODSP router
c. An internal router
d. An intra-area router
11. How many IS-IS processes run on a Level 1-2 router?
c. One for each area
d. Two for each area
12. A Level 2 router is similar to which type of OSPF router? a. ASBR
d. Backbone internal router
13. Where are IS-IS areas deﬁned?
a. On the link
b. On the interface
c. At the process
d. A separate process is required for each routing table
14. Which of the following statements is true about Level 2 routers?
a. Level 2 routers can send updates between routing areas.
b. They must be placed contiguously.
c. They are similar to OSPF stub routers.
d. This is the Cisco default conﬁguration.
15. Once a datagram is accepted into the router, which of the following ﬁelds are stripped?
b. System ID
d. Protocol type
16. For an adjacency to be formed and maintained, both interfaces must agree on which of the following?
a. If the routers are both Level 1, they must be in the same area.
b. The system ID must be unique to each router.
c. Each router needs to be conﬁgured as the same level of routing—that is, either Level 1 or Level 2.
d. The Hello timers.
17. How does the pseudonode represent all the routers on the LAN?
a. Each router on the multiaccess LAN simulates an interface on the pseudonode.
b. The pseudonode represents the physical link to which the routers are connected.
c. The pseudonode is the virtual link between two discontiguous areas.
d. Each link on every router on the LAN is summarized to form a pseudonode for the LAN.
18. What is used to elect the DIS automatically?
a. Highest SNPA address.
b. Priority deﬁned at the interface.
c. The DIS must be manually conﬁgured.
d. The highest loopback address.
19. Which of the following packets are used by IS-IS?
20. Which of the following are valid hello packet types for IS-IS?
a. Hello Level 1 LAN
b. Hello Level 2 LAN
c. Hello Level 1-2 LAN
d. Hello Point to Point
21. 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?
a. A generic Hello
b. Point-to-point Level 1, point-to-point Level 2, LAN Level 1, LAN Level 2
c. Point-to-point, LAN Level 1, LAN Level 2
d. Point-to-point Level 1, point-to-point Level 2, LAN
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:
■ 12 or less overall score —Read the entire chapter. This includes the “Foundation Topics,” “Foundation Summary,” and “Q&A” sections at the end of the chapter.
■ 13–18 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.
Introduction to Integrated IS-IS
IS-IS is an Interior Gateway Protocol (IGP) developed in the 1980s by Digital Equipment and submitted to the International Organization for Standardizaton (ISO) as the routing protocol for Open System Interconnection (OSI). The creation of IS-IS was an attempt to produce a standard protocol suite that could allow internetworks to scale.
The development of IS-IS was motivated by the need for the following:
■ A nonproprietary protocol
■ A large addressing scheme
■ A hierarchical addressing scheme
■ A protocol that was efﬁcient, allowing for fast, accurate convergence and low network overhead The United States mandated that every system operated by the government had to be capable of running the OSI architecture (as an initiative called the Government Open Systems Interconnections Proﬁle [GOSIP]). By forcing every government system to understand OSI, ofﬁcials hoped that the protocols would become the practical standard and the academic solution to diverse proprietary implementations for computer networking. In the end, however, the initiative failed. The Internet, built on TCP/IP, prevailed as the practical alternative to an international standard. However, IS-IS has always been used, although not as extensively as ﬁrst hoped. Large Internet service providers (ISPs) have been using IS-IS since its inception in the 1980s, and recently IS-IS has begun to emerge in other markets. This new interest is for a variety of reasons, including the fact that IS-IS is a
standard that provides protocol independence, it has the capability to scale, and it has the capacity to deﬁne type of service (ToS) routing, though currently this is not a supported feature in the Cisco IOS. ToS routing allows trafﬁc engineering, which requires very complex routing decisions to be programmed into the protocol. Therefore, as of late, IS-IS has been taken from the shelf, dusted off, and put into use.
The terminology used by IS-IS might appear cumbersome and unfriendly. Although the jargon might be unfamiliar, most of the concepts are no different from other routing protocols. Table 10-2 explains brieﬂy some of the commonly used IS-IS terms.
Table 10-2 IS-IS Terms
Introduction to Integrated IS-IS 343
Table 10-2 IS-IS Terms (Continued)
344 Chapter 10: Fundamentals of the Integrated IS-IS Protocol
Table 10-2 IS-IS Terms (Continued)
Introduction to Integrated IS-IS 345
Table 10-2 IS-IS Terms (Continued)
346 Chapter 10: Fundamentals of the Integrated IS-IS Protocol
Table 10-2 IS-IS Terms (Continued)
OSPF and IS-IS Compared
Integrated IS-IS and OSPF share a common heritage. The following sections compare the similarities and differences between the two routing protocols.
Similarities Between Integrated IS-IS and OSPF
They are both link-state protocols and are based on the Dijkstra algorithm of Shortest Path First (SPF). In addition, they both have a two-level hierarchy. OSPF tends to be deployed mostly as an enterprise solution, whereas Integrated IS-IS is used for IP routing in some ISP networks.
Table 10-3 outlines other similarities between Integrated IS-IS and OSPF.
Table 10-3 Integrated IS-IS and OSPF Similarities and Terminology Comparison
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Table 10-3 Integrated IS-IS and OSPF Similarities and Terminology Comparison (Continued)
Differences Between OSPF and Integrated IS-IS
Although OSPF and Integrated IS-IS share the same common goals and use the same link-state technology to achieve those goals, the methods they use differ slightly. For example, the protocols differ in how the area address is assigned. In IS-IS, the area and host address are assigned to the entire router, whereas in OSPF, the address is assigned at the interface level. An IS-IS router is therefore in one area, while in OSPF, a router can inhabit many areas, though there is an option for multiarea IS-IS that is used primarily during area migrations and transition.
This means that all Level 1 routers have to be within the same area, with a Level 1-2 router connecting them to another area. However, the Level 1-2 router needs to be in the same area as the Level 1 router with which it communicates. The Level 1-2 router can see the rest of the autonomous system and offers itself as the default route to the Level 1 area. This is similar to the OSPF stub areas. The Level 2 router sends Level 2 updates in the other area, or preﬁx routes, just like the ABR in OSPF.
The role of the DR is subtly different. The DIS in IS-IS exists for both Level 1 and Level 2 on multiaccess media, but there is no backup designated router (BDR). Also, in OSPF the DR is elected for life; in IS-IS, however, if another router comes on line with a higher priority, the existing DIS is
deposed. Fewer adjacencies are formed in OSPF because the routers form adjacencies only with the DR and the BDR. In IS-IS, every router makes an adjacency with every other router on the medium. However, IS-IS LSPs are sent out only by the DIS on behalf of the pseudonode.
A major difference is the encapsulation of the two protocols. IS-IS has protocol independence because it runs directly on top of the data-link layer. Fragmentation is the responsibility of the IS-IS process, and this allows for a streamlined protocol. More importantly, fragmentation makes evolution of the protocol as needed very simple, because it is not dependent and therefore is not limited by any third protocol. OSPF is encapsulated into IP and is limited by the capabilities of that protocol.
The way that the LSPs are handled is also slightly different and inﬂuence, to an extent, the design of a network running either protocol. Unrecognized LSPs are ignored and ﬂooded in IS-IS, for example; OSPF ignores and drops unrecognized LSAs.
Table 10-4 lists the key differences.
Table 10-4 Integrated IS-IS and OSPF Technical Differences
350 Chapter 10: Fundamentals of the Integrated IS-IS Protocol
Table 10-4 Integrated IS-IS and OSPF Technical Differences (Continued)