IS-IS

Intermediate System to Intermediate System (IS-IS) is a designed to move information efficiently within a computer network, a group of physically connected computers or similar devices. It accomplishes this by determining the best route for datagrams through a packet-switched network.

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" by RFC 7142 because it re-published an ISO draft rather than the ISO standard, causing industry confusion

IS-IS has been called "the de facto standard for large service provider network backbones."

IS-IS (pronounced by speaking the name of each letter: "i-s i-s"; that is, "eye-ess eye-ess") is an , designed for use within an administrative domain or network. This is in contrast to , primarily (BGP), which is used for routing between autonomous systems (RFC 1930).

IS-IS is a , operating by reliably flooding link state information throughout a network of routers. Each IS-IS router independently builds a database of the network's topology, aggregating the flooded network information. Like the OSPF protocol, IS-IS uses Dijkstra's algorithm for computing the best path through the network. Packets (datagrams) are then forwarded, based on the computed ideal path, through the network to the destination.

The IS-IS protocol was developed by Digital Equipment Corporation as part of DECnet Phase V. It was standardized by the ISO in 1992 as ISO 10589 for communication between network devices which are termed Intermediate Systems (as opposed to end systems or hosts) by the ISO. The purpose of IS-IS was to make possible the routing of datagrams using the ISO-developed OSI called CLNS.

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