This chapter elaborated on the architecture of the IS-IS routing protocol, discussing basic concepts as well as advanced protocol mechanisms involving the link-state database. The chapter also provided insight into configuration procedures required for enabling IS-IS routing on Cisco routers. Even though this book is focused on use of IS-IS for IP routing, some time was dedicated to exploring the origins of the IS-IS protocol as a dynamic routing application for ISO CLNP. IS-IS is specified in ISO 10589, which is reproduced as RFC 1142. RFC 1195 adapts IS-IS for IP routing by introducing extensions (TLV fields) for carrying IP routing information in addition to CLNP information.

CLNP addresses, also called NSAPs, are different from IP addresses: They have a variable length from 8 bytes (on Cisco routers) up to 20 bytes, compared to the fixed 4-byte length for IP addresses. Also, NSAPs are node-based, while IP addresses are configured on router inter-faces, essentially numbering the connected links (link-based). You also learned in this chapter about two types of links commonly recognized in IS-IS implementations: point-to-point and broadcast links. These two links types are tied to the two types of adjacencies supported in IS-IS: point-to-point and broadcast adjacencies. IS-IS adjacencies are needed for subsequent sharing of link-state information and building of link-state databases on participating routers. IS-IS hello packets are used to establish and maintain adjacencies.

IS-IS supports a two-level routing hierarchy with level-1 routing occurring in sections of the network referred to as areas. Areas constitute interconnected routers with a common area identifier in their NSAP addresses. The region that spans interconnection between areas is a special area known as the IS-IS backbone. Level 2 routing occurs in the backbone. The special packets for advertising routing information between adjacent IS-IS routers are link-state packets. Flooding is the process used in transmitting LSPs between routers. Routers in the same area must have the same Level 1 link-state database; similarly, routers in the back-bone must have the same Level 2 link-state database. The process for ensuring consistency in the various link-state databases between routers is database synchronization. Special packets known as sequence number packets (CSNPs and PSNPs) are used for the synchron-ization process. You also learned how sequence number packets are used for database synchronization.

In discussing the IS-IS configuration on Cisco routers, examples for serial point-to-point links and ATM connectivity are provided. In addition, it is noted that the examples provide baseline configuration applicable to other media, such as Frame Relay. Enabling IS-IS routing on a Cisco router involves two basic steps: configuring the IS-IS routing process and then enabling IS-IS routing on the interfaces where IS-IS adjacencies and route sharing occur.

This chapter provides a review of the IS-IS protocol and prepares you for the next chapter, which discusses techniques for troubleshooting IS-IS routing problems. For more complete coverage on configuring the IS-IS routing protocol on Cisco routers, reading references are provided at the end of the chapter.


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