Intermediate System to Intermediate System

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 data through a packet-switched network.

The IS-IS protocol is defined in ISO/IEC 10589:2002 as an international standard within the Open Systems Interconnection (OSI) reference design. The Internet Engineering Task Force (IETF) republished IS-IS in RFC 1142, but that RFC was later marked as "historic" by RFC 7142 because it republished a draft rather than a final version of the ISO standard, causing 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 pair; that is, "iz iz") 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 that 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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