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This article is about a computer network protocol. For topics with similar names, see Isis (disambiguation).

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.

IS-IS has been called "the de facto standard for large service provider network backbones."[1]


IS-IS (pronounced "i-s i-s") 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 which 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)

Comparison with OSPF

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.[2] It is at the same layer as Connectionless Network Service (CLNS). The widespread adoption of IP may have contributed to OSPF's popularity. IS-IS does not use IP to carry routing information messages. IS-IS is neutral regarding the type of network addresses for which it can route. OSPF version 2, on the other hand, was designed for IPv4. 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).

IS-IS routers build a topological representation of the network. This map indicates the subnets which each IS-IS router can reach, and the lowest-cost (shortest) path to a subnet is used to forward traffic.





OSPF has a larger set of extensions and optional features specified in the protocol standards. However IS-IS is more easy to expand: its use of type-length-value (TLV) data allows engineers to implement support for new techniques without redesigning the protocol. For example, in order to support IPv6, the IS-IS protocol was extended to support a few additional TLVs, whereas OSPF required a new protocol draft (OSPFv3). In addition to that, IS-IS is less "chatty" and can scale to support larger networks. Given the same set of resources, IS-IS can support more routers in an area than OSPF. This has contributed to IS-IS as an ISP-scale protocol.[citation needed]


Other Uses

IS-IS is also used as the control plane for IEEE 802.1aq Shortest Path Bridging (SPB). SPB allows for shortest-path forwarding in an Ethernet mesh network context utilizing multiple equal cost paths. This permits SPB to support large Layer 2 topologies, with fast convergence, and improved use of the mesh topology.[3] Combined with this is single point provisioning for logical connectivity membership. IS-IS is therefore augmented with a small number of TLVs and sub-TLVs, and supports two Ethernet encapsulating data paths, 802.1ad Provider Bridges and 802.1ah Provider Backbone Bridges. SPB requires no state machine or other substantive changes to IS-IS, and simply requires a new Network Layer Protocol Identifier (NLPID) and set of TLVs. This extension to IS-IS is defined in the IETF proposed standard RFC 6329.

Related protocols


  1. Gredler, Hannes; Goraiski, Walter (2005). The complete IS-IS routing protocol. Springer. p. 1. ISBN 1-85233-822-9. 
  2. Page 80
  3. "IS-IS Extensions Supporting IEEE 802.1aq Shortest Path Bridging". IETF. April 2012. 

External links