IS-IS

Intermediate System to Intermediate System (IS-IS, also written ISIS) 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 data through a packet-switched network. Intermediate System to Intermediate System (IS-IS, also written ISIS) 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 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 is an interior gateway protocol, designed for use within an administrative domain or network. This is in contrast to exterior gateway protocols, primarily Border Gateway Protocol (BGP), which is used for routing between autonomous systems (RFC 1930). IS-IS is a link-state routing protocol, 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 protocol stack called CLNS. IS-IS was developed at roughly the same time that the Internet Engineering Task Force IETF was developing a similar protocol called OSPF. IS-IS was later extended to support routing of datagrams in the Internet Protocol (IP), the Network Layer protocol of the global Internet. This version of the IS-IS routing protocol was then called Integrated IS-IS (RFC 1195) Both IS-IS and Open Shortest Path First (OSPF) are link state protocols, and both use the same Dijkstra algorithm for computing the best path through the network. As a result, they are conceptually similar. Both support variable length subnet masks, can use multicast to discover neighboring routers using hello packets, and can support authentication of routing updates. While OSPF was natively built to route IP and is itself a Layer 3 protocol that runs on top of IP, IS-IS is an OSI Layer 2 protocol. It is at the same layer as Connectionless Network Protocol (CLNP). The widespread adoption of IP may have contributed to OSPF's popularity. IS-IS does not use IP to carry routing information messages. OSPF version 2, on the other hand, was designed for IPv4. IS-IS is neutral regarding the type of network addresses for which it can route. This allowed IS-IS to be easily used to support IPv6. To operate with IPv6 networks, the OSPF protocol was rewritten in OSPF v3 (as specified in RFC 2740).