IS-IS is a link state protocol that distributes routing information between routers belonging to a single IP domain; the IP domain is also known as an autonomous system (AS). In a link-state routing protocol, each router maintains a database describing the topology of the AS. Each participating router has an identical database maintained from the perspective of that router.
From the link state database (LSDB), each router constructs a tree of shortest paths, using itself as the root. The shortest path tree provides the route to each destination in the AS. When several equal-cost routes to a destination exist, traffic can be distributed among them. The cost of a route is described by a single metric.
IS-IS is an interior gateway protocol (IGP), as are RIP and OSPF. Unlike RIP and OSPF, IS-IS was not initially designed for IP. RFC 1195 specifies how IS-IS can run in an IP environment. The Extreme Networks implementation supports IS-IS only in IP environments. RIP, OSPF, and IS-IS are compared in RIP. The IPv6 versions of these protocols are compared in RIPng.