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, the ISO (International Organization for Standardization) also created something similar to IPv4 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.