IS-IS is an IGP, link-state routing protocol, similar to OSPF. It forms neighbor adjacencies, has areas, exchanges link-state packets, builds a link-state database, and runs the Dijkstra SPF algorithm to find the best path to each destination, which is installed in the routing table.

Back when OSPF and IS-IS were developed, IP wasn’t the dominant protocol that it is today. When people think of OSI they automatically think of the OSI-model but back then, ISO (International Organization for Standardization) also created something similar to IP and UDP called CLNP (Connectionless-mode Network Protocol) and CLNS (Connectionless-mode Network Service).

ISO also uses some different terminology, for example:

  • Router = Intermediate system
  • Host = End system

Unlike OSPF which was developed by the IETF (Internet Engineering Task Force), IS-IS was originally developed by DEC for CLNS, not IP and this is why it’s called IS-IS (Intermediate System – Intermediate System).

Later, IS-IS was adapted so that it could also route IP and is then called integrated IS-IS.