Intermediate System to Intermediate System (IS-IS) is a link status protocol that uses the shortest path first (SPF) algorithm to calculate routes.
The Intermediate System-to-Intermediate System (IS-IS) is a dynamic routing protocol initially designed by the International Organization for Standardization (ISO) for its Connectionless Network Protocol (CLNP).
To support IP routing, the Internet Engineering Task Force (IETF) extends and modifies IS-IS in RFC 1195. This enables IS-IS to be applied to TCP/IP and OSI environments. This type of IS-IS is called Integrated IS-IS or Dual IS-IS.
IS-IS stated in this document refers to Integrated IS-IS, unless otherwise stated.
As an Interior Gateway Protocol (IGP), IS-IS is used in Autonomous Systems (ASs). IS-IS is a link state protocol. It uses the Shortest Path First (SPF) algorithm to calculate routes.