Intermediate System to Intermediate System (IS-IS) is an interior gateway protocol for routing IP packets. The protocol was defined in ISO/IEC 10589:2002 as an international standard within the Open Systems Interconnection (OSI) reference design.
Though originally an ISO standard, the IETF republished the protocol in RFC 1142. RFC 1142 was reclassified as “historic” because it re-published an ISO draft rather than the ISO standard, causing industry confusion.
Allows routers to exchange information about available routes and update their routing tables, which they then use to decide where to send each IP packet which arrives. Fast Convergence compared to old protocols like RIP. It is similar to OSPF (another routing protocol) in what it does but uses different terminology. IS-IS information is carried directly over layer 2 (often Ethernet). It is most commonly used by ISPs (internally) and providers of network backbones. Smaller organizations more commonly use OSPF.
See Router, OSPF
