IS-IS is a link-state interior gateway routing protocol. Like OSPF, IS-IS runs the Dijkstra shortest-path first (SPF) algorithm to create a database of the network's topology and, from that database, to determine the best (that is, shortest) path to a destination.
Unlike OSPF, which was developed and standardized by the Internet Engineering Task Force (IETF), IS-IS is an ANSI ISO protocol and was originally based on the Digital Equipment Corporation DECNET Phase V Network Technology.
IS-IS uses a slightly different terminology than OSPF for naming its protocol packets. The packets that IS-IS routers send to each other describing the network topology are called link-state protocol data units (link-state PDUs, or LSPs). In addition to describing the network topology that the router knows about, the link-state PDUs include IP routes, checksums, and other information.
Similar to OSPF, all IS-IS routers place the information in the received link-stated PDUs into their link-state database, and all routers have the same view of the network's topology. IS-IS runs the SPF algorithm on the information in the link-state database to determine the shortest path to each destination on the network, placing the destination/next-hop pairs that result from the SPF calculation into the IS-IS routing database.
Unlike other IP routing protocols, which typically run on TCP, UDP, or IP, which are OSI Layer 3 or Layer 4 protocols, IS-IS runs directly ...