IS-IS
From Wikipedia, the free encyclopedia
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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[edit] 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 an Exterior Gateway Protocol, such as 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.
[edit] History
IS-IS was developed by Digital Equipment Corporation and later handed over to the ISO 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 using the IP, the basic protocol carried on the Internet. This is then called Integrated IS-IS.
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.
[edit] 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 neighboring routers using hello packets, and can support authentication of routing updates.
While OSPF is natively built to route IP and is itself a layer 3 protocol that runs on top of IP, IS-IS is natively an ISO CLNS protocol, a fact that may have allowed OSPF to be more widely used. 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, known as "Integrated IS-IS" or "Dual IS-IS", is described in RFC 1195. IS-IS was originally described in ISO 10589, in 1992, and was based on DECnet Phase V.
IS-IS differs from OSPF in the way that "areas" are defined and routed between. IS-IS routers are designated as being Level 1 (intra-area), Level 2 (interarea) or Level 1-2 (both). Level 2 routers are interarea routers that can only form relationships with other Level 2 routers. Routing information is exchanged between Level 1 routers and other Level 1 routers and Level 2 routers only exchange information with other Level 2 routers. Level 1-2 routers exchange information with both levels and are used to connect the interarea routers with the intra area routers Whereas in OSPF areas are delineated on the interface such that an area border router (ABR) is actually in two or more areas at once, effectively creating the borders between areas, inside an ABR, IS-IS area borders are in between routers, designated as Level 2 or Level 1-2. The result is that an IS-IS router is only ever a part of a single area. IS-IS also does not require Area 0 (Area Zero) to be the backbone area through which all inter-area traffic must pass. The logical view is that OSPF creates something of a spider web or star topology of many areas all attached directly to Area Zero and IS-IS by contrast creates a logical topology of something akin to a vertebrate of Level 2 routers with branches of Level 1-2 and Level 1 routers forming the individual areas.
