IS-IS
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Intermediate system to intermediate system (IS-IS), is a protocol used by network devices (routers) to determine the best way to forward datagrams or packets through a packet-based network, a process called routing.
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Description
IS-IS is an Interior Gateway Protocol (IGP) meaning that it is intended for use within an administrative domain or network. It is not intended for routing between networks or administrative domains, a job which is the purpose of the Border Gateway Protocol (BGP).
IS-IS is a link-state routing protocol, meaning that it operates by reliably flooding topology information throughout a network of routers. Each router then independently builds a picture of the network's topology. Packets or datagrams are forwarded based on the best topological path through the network to the destination.
IS-IS uses Dijkstra's algorithm for identifying the best path through the network.
History
IS-IS was developed by the International Telecommunications Union for communication between network devices which are termed Intermediate Systems (as opposed to end systems or hosts) by the ITU. The purpose of IS-IS was to make possible the routing of datagrams using the ITU-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 using the IP, the basic protocol carried on the Internet.
OSPF achieved dominance among IGP routing protocols, particularly in Enterprise networks. IS-IS, in contrast, remained largely unknown by most network engineers and was used predominantly in the networks of large service providers.
IS-IS has become more widely known in the last several years, and has become a viable alternative to OSPF in the enterprise.
Comparison with OSPF
Both IS-IS and 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 neighbouring routers using hello packets, and can support authentication of routing updates.
While OSPF is natively built to route IP and is itself a layer 4 protocol that runs on top of IP, IS-IS is natively an ISO CLNS protocol. IS-IS does not use IP to carry routing information messages.
IS-IS routers build a topological representation of the network. This map indicates the IP subnets which each IS-IS router can reach, and the lowest cost (shortest) path to an IP subnet is used to forward IP traffic.
IS-IS also differs from OSPF in the methods by which it reliably floods topology and topology change information through the network. However, the basic concepts are similar.
Since OSPF is more popular, this protocol has a richer set of extensions and added features. Many claim, however, that IS-IS is less "chatty" and can scale to support larger networks. Additionally, IS-IS is much more neutral regarding the type of network addresses it can route for. OSPF, on the other hand, was very much built with IPv4 in mind. Thus IS-IS was easily adapted to support IPv6, while the OSPF protocol needed a major overhaul (OSPF v3).
The TCP/IP implementation was described in RFC 1195. IS-IS was originally described in ISO 10589.
External links
French article about Cisco IS-IS (http://www.labo-cisco.com/ArticleComp.asp?ARID=61)de:IS-IS fr:IS-IS nl:Intermediate System-to-Intermediate System pl:IS-IS