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IS-IS (Intermediate System-to-Intermediate System) Routing

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Note: Many topics at this site are reduced versions of the text in "The Encyclopedia of Networking and Telecommunications." Search results will not be as extensive as a search of the book's CD-ROM.

IS-IS is an OSI (Open Systems Interconnection) link-state routing protocol that dynamically routes packets between routers or intermediate systems. OSPF (Open Shortest Path First) is an Internet link-state protocol based on IS-IS.

IS-IS (and OSPF) is an interior (intradomain) routing protocol designed to work within an autonomous system (AS). The corresponding OSI exterior (interdomain) routing protocol is IDRP (Interdomain Routing Protocol), which is designed to exchange routing information between autonomous systems. IDRP is based on the Internet BGP (Border Gateway Protocol) exterior routing protocol.

IS-IS routers flood an internetwork with link-state information. Other routers receive this information and build a database that describes all the routes on the network. A routing table is then calculated from this information.

IS-IS supports a two-level hierarchy in which groups of routers can be delegated to an area (domain). Routing information is then contained within the domain and summary information for those routes is exchanged with other areas. Some Internet documents that provide information about IS-IS are listed here:

  • RFC 1074 (The NSFNET Backbone SPF Based Interior Gateway Protocol, October 1988)

  • RFC 1195 (Use of OSI IS-IS for Routing in TCP/IP and Dual Environments, December 1990)

  • RFC 2763 (Dynamic Hostname Exchange Mechanism for IS-IS, February 2000)

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