Intermediate System-Intermediate System

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Intermediate System-Intermediate System (IS-IS) is one of the two nonproprietary and highly scalable Internet interior routing protocols, the other being Open Shortest Path First. Technically, the version used in the Internet, to carry routing information about Internet Protocol version 4 addresses, is Integrated IS-IS, as ISIS was originally developed only to carry information about the Connectionless Network Protocol (CLNP) of the Open System Interconnection protocol suite. Integrated IS-IS is specified in RFC 1195.

It was originally developed by Radia Perlman for the Digital Equipment Corporation's DECNET Phase V routing, and then submitted to the International Organization for Standardization as the interior routing protocol for OSI protocol (i.e., not only the Open Systems Interconnection Reference Model (OSIRM) architecture, but OSI-specific protocols within that architecture. Ross Callon extended its design to be able to carry IP as well as OSI information. ISIS has also been extended to support IPv6 routing / route exchange, currently an IETF draft (but widely supported nonetheless). With regards to IPv6 support, note that ISIS supports both "Single Topology (Single-SPF, multi-protocol)" as well as "MultiTopology (Multi-SPF, single protocol) modes of operation.

While IS-IS and OSPF share many principles, such as link state route computation, there are a great many detailed differences. In practice, this means that IS-IS tends to be the best choice for very large, but stable and homogeneous, networks such as is found in service provider networks. IS-IS is also designed to be easily extended with new features, while OSPF was optimized for processing efficiency.