Intermediate System to Intermediate System (IS-IS) Routing Protocol
Intermediate System to Intermediate System (IS-IS) is link-state routing protocol, and its technical information is found in RFC1142. It was developed by Digital Equipment Company for DECNet at the same time Internet Engineering Task Force (IETF) was working on OSPF for IP. As such, they do share some similarities in purpose. Since being developed, IS-IS has been extended to support IP and was published as an open standard in RFC1195.
IS-IS is used primarily by service providers on their network to route data, so you will not likely see it or have to work with it. One major difference between IS-IS and other routing protocols is that IS-IS does not rely on IP to distribute routing information traffic.
IS-IS, which is independent of the network layer protocols being used, uses any network layer protocol to send its messages; in addition, IS-IS can distribute routing information for any protocol, not just IPv4, which simplified updating the protocol to support IPv6.
IS-IS works with two basic areas, a Level 1 (intra-area) or Level 2 (inter-area). A third area designator is a hybrid Level 1–2 (both areas). Each IS-IS router is only ever part of an area, but Level 1–2 routers can exchange information with routers in both areas. Within an area, the methods that IS-IS uses to manage routing information is similar to that of OSPF.