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". The protocol was defined in RFC 1142 as a protocol within the Open Systems Interconnection (OSI) reference design. IS-IS is not an Internet standard.


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 Autonomous Systems (RFC 1930), a job that 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.


The IS-IS protocol was developed by Digital Equipment Corporation as part of DECnet Phase V. It was standardised 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 (aka network-layer packets) using IP Protocol, the basic routed protocol of the global (public) Internet. This version of the IS-IS routing protocol was then called Integrated IS-IS ( [ RFC 1195] ).

OSPF had achieved predominance as an IGP (Interior Gateway Protocol) routing protocol, particularly in medium-to-large-sized enterprise networks. IS-IS, in contrast, remained largely unknown by most network engineers and was used predominantly in the networks of certain very-large service providers.

IS-IS has become more widely known in the last several years, and has become a viable alternative to OSPF in enterprise networks. Detailed analysisFact|date=March 2008, however, tends to show that OSPF has traffic tuning features that are especially suitable to enterprise networks while ISIS has stability features especially suitable to ISP infrastructure.

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 network layer protocol (it is at the same layer as CLNS), 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. However 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 makes IS-IS favoured in ISP environments. Additionally, IS-IS is neutral regarding the type of network addresses for which it can route. OSPF, on the other hand, was designed for IPv4. 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 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 (inter area); or Level 1-2 (both). Level 2 routers are inter area 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 inter area routers with the intra area routers. 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 the ABR, whereas in 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 a backbone of Level 2 routers with branches of Level 1-2 and Level 1 routers forming the individual areas.

Other related protocols

Fabric Shortest Path First (FSPF):
* Level 0: Between ES's and IS's on the same subnet. OSI routing begins at this level; END-System and Intermediate System
* Level 1: Between IS's on the SAME AREA. Also called area routing/intra-area
* Level 2: Called Inter-Area routing
* Level 3: Routing between separate domains; It is similar to BGP


* [ 6WINDGate - 6WIND] , commercial embedded open-source routing modules
*Quagga, a fork of GNU Zebra for Unix-like systems


* RFC 1142 - IS-IS protocol specification (IETF) - Note: this is a copy of DP 10589 (Draft Proposal) and differs in many significant details from the final version of ISO/IEC 10589
* RFC 1195 - Use of OSI IS-IS for Routing in TCP/IP and Dual Environments

External links

* [ ISO/IEC 10589:2002 Second Edition]
* [ Collection of RFCs pertaining to IS-IS]
* [ Configuring integrated IS-IS on Cisco Routers]
* [ IS-IS and OSPF difference discussion] (Vishwas Manral, Manav Bhatia and Yasuhiro Ohara)
* [ Sample isisd.conf file] : used with Quagga

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