Chapter Summary

The IS-IS routing protocol is one of three network layer protocols specified by ISO to support the CLNS. The others are CLNP and ES-IS. Even though IS-IS was designed for routing ISO CLNP packets, it has been adapted for use in IP environments as Integrated IS-IS.

Integrated IS-IS has evolved over the years into a scalable, robust, and easy-to-use Interior Gateway Routing Protocol (IGP) that can operate in dual mode to support overlayed IP and ISO CLNS networks. However, IS-IS has probably gained more popularity on the Internet, where its primary application is for intradomain IP routing. As an IGP in an Internet routing domain or autonomous system, Integrated IS-IS plays a critical supportive role for the Border Gateway Protocol (BGP). BGP is designed with more elaborate policy-handling capabilities and can handle a significantly large number of routes of the scale that can be found on the Internet.

IS-IS is a link-state protocol and therefore gathers routing information from adjacent neighbors into a link-state database and uses the SPF algorithm (named after Dijkstra) to determine the best paths to destinations within the network.

In recent years, the Integrated IS-IS has been further enhanced to handle emerging networking technologies, such as Traffic Engineering with Multiprotocol Label Switching (MPLS/TE).