The Intermediate System-to-Intermediate System (IS-IS) protocol is an interior gateway protocol (IGP) that uses link-state information to make routing decisions.
IS-IS is a link-state IGP that uses the shortest-path-first (SPF) algorithm to determine routes. This protocol originally was developed for routing International Organization for Standardization (ISO) Connectionless Network Protocol (CLNP) packets.
This chapter discusses the following topics that provide background information about IS-IS: