Summary

This chapter covers one of the key underlying concepts of the IS-IS routing protocol: the IS-IS Link-State database. First, the functional organization of the IS-IS Link-State database is discussed. In support of the two-level routing hierarchy, each router supports a Level 1 and a Level 2 database for Level 1 and Level 2 routing, respectively. Subsequent discussions in the chapter focus on elements stored in the database (LSPs) and the associated control processes and elements that help in building and maintenance of the distributed Link-State database. Sequence number packets (CSNP and PSNP) and various timers used in related control processes ensure the integrity and consistency of the LSPs stored in the Level 1 and Level 2 Link-State databases across multiple routers.

Each Level 1 database describes the topological organization of the corresponding area in the domain, whereas a single Level 2 database captures the overall layout of the Level 1 areas in relation to each other. Consequently, all the routers in an area build the same Level 1 database for the area and attain a consistent view of the intra-area topology. In a similar manner, Level 2 routers connecting to the backbone build identical Level 2 databases. This is a critical requirement for each router to calculate loop-free paths to all known destinations in the network. Each router feeds the information in the Link-State database as input to the SPF algorithm for computation of best paths. Chapter 6 provides detailed coverage of the SPF algorithm.

LSP copies are distributed in the Level 1 areas and the backbone by a mechanism referred to as flooding. Control mechanisms involving CSNPs, PSNPs, SRM and SSN flags, and various timers are employed to assist flooding of LSPs and synchronization of the Link-State database between routers.

This chapter covers packet formats of LSPs, CSNPs, and PSNPs, providing details about fields in the headers, as well as the TLVs supported by these packets. Flooding operations are discussed and elaborated on for point-to-point and broadcast links and recommendations are made for dealing with NBMA links. Mesh groups provide an alternative for handling excessive and redundant flooding in highly meshed NBMA environments.

This chapter also covers IS-IS metrics, noting that metric information is carried in specific LSP TLVs. The IS-IS protocol supports four types of metrics, of which only the default metric is implemented by most router vendors , including Cisco Systems. The metrics discussion reviews recent extensions to the IS-IS protocol within the IETF to support larger metric values. These metric-related enhancements are designed to overcome limitationsof the default metric, which was originally specified to have a maximum of only 63 per interface and 1023 for an entire path . The Extended IS Reachability and Extended IP Reachability TLVs, which were recently proposed in the IETF, are designed to address these limitations.

In general, operation of routing protocols is automated and guided by many protocol timers. The IS-IS protocol has its fair share of timers (as indicated previously in relation to the Link-State database). This chapter concludes with an overview of various critical timers, which are key to the operation of IS-IS and are ultimately responsible for routing stability.