Intermediate System-Intermediate System

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Intermediate System-Intermediate System (IS-IS) is one of the two nonproprietary and highly scalable Internet routing protocol#interior routing protocol|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.