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'''Intermediate System to Intermediate System ''' (IS-IS) is a routing protocol 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."