Intermediate System to Intermediate System Protocol (IS-IS) allows routing of ISO’s OSI protocol stack Connectionless Network Service (CLNS). IS-IS is an Interior Gateway Protocol (IGP) not intended to be used between Autonomous Systems (ASes).
IS-IS was developed by Digital Equipment Corporation and later standardized by ISO in 1992 as ISO 19589 (see RFC 1142—note this RFC is different from the ISO version). At roughly the same time, the Internet Engineering Task Force developed OSPF (see Open Shortest Path First (OSPF)). After the initial version, IP support was added to IS-IS and this version was called Integrated IS-IS (see RFC 1195). Its widespread use started when an early version of IS-IS was included with BSD v4.3 Linux as the routed daemon. The routing algorithm used by IS-IS, the Bellman–Ford algorithm, first saw widespread use as the initial routing algorithm of the ARPANET.
IS-IS is a link state protocol well-suited to smaller networks that is in widespread use and has near universal support on routing hardware. It is quick to configure, and works well if there are no redundant paths. However, IS-IS updates are sent out node-by-node, so it can be slow to find a path around network outages. IS-IS also lacks good authentication, can not choose routes based on different quality of service methods, and can create network loops if you are not careful. IS-IS uses Djikstra’s algorithm to find the best path, like OSPF.
While OSPF is more widely known, IS-IS is a viable alternative to OSPF in enterprise networks and ISP infrastructures, largely due to its native support for IPv6 and its non-disruptive methods for splitting, merging, migrating, and renumbering network areas.