Routing protocol

A routing protocol is a protocol that specifies how routers communicate with each other, disseminating information that enables them to select routes between any two nodes on a computer network, the choice of the route being done by routing algorithms. Each router has a priori knowledge only of networks attached to it directly. A routing protocol shares this information first among immediate neighbors, and then throughout the network. This way, routers gain knowledge of the topology of the network. For a discussion of the concepts behind routing protocols, see: Routing.

The term routing protocol may refer specifically to one operating at layer three of the OSI model, which similarly disseminates topology information between routers.

Although there are many types of routing protocols, three major classes are in widespread use on IP networks:

• Interior gateway routing via link state routing protocols, such as OSPF and IS-IS
• Interior gateway routing via path vector or distance vector protocols, such as IGRP and EIGRP
• Exterior gateway routing. BGP v4 is the routing protocol used by the public Internet.

Many routing protocols are defined in documents called RFCs.^[1][2][3][4]

The specific characteristics of routing protocols include

• the manner in which they either prevent routing loops from forming or break them up if they do
• the manner in which they select preferred routes, using information about hop costs
• the time they take to converge
• how well they scale up
• many other factors

Routed versus routing protocols

A routed protocol can be routed by a router, i.e., it can be forwarded from one router to another. A routing protocol sends and receives packets containing routing information to and from other routers.

In some cases, routing protocols can themselves run over routed protocols: for example, BGP runs over TCP which runs over IP; care is taken in the implementation of such systems not to create a circular dependency between the routing and routed protocols. That a routing protocol runs over particular transport mechanism does not mean that the routing protocol is of layer (N+1) if the transport mechanism is of layer (N). Routing protocols, according to the OSI Routing framework, are layer management protocols for the network layer, regardless of their transport mechanism:

• IS-IS runs over the data link layer
• OSPF, IGRP, and EIGRP run directly over IP; OSPF and EIGRP have their own reliable transmission mechanism while IGRP assumed an unreliable transport
• RIP runs over UDP
• BGP runs over TCP

Interior routing protocols

Interior Gateway Protocols (IGPs) exchange routing information within a single routing domain. Examples of IGPs include:

• OSPF (Open Shortest Path First)
• RIP (Routing Information Protocol)
• IS-IS (Intermediate System to Intermediate System)
• EIGRP (Cisco's proprietary Enhanced Interior Gateway Routing Protocol)

Note that IGRP, a Cisco proprietary routing protocol, is no longer supported. EIGRP accepts IGRP configuration commands, but the internals of IGRP and EIGRP are completely different.

See also

• Routing
• Static routing
• Adaptive routing

References

1. ^ INTERNET PROTOCOL, RFC 791, J Postel, September 1981.
2. ^ BROADCASTING INTERNET DATAGRAMS IN THE PRESENCE OF SUBNETS, RFC 922, Jeffrey Mogul, October 1984
3. ^ Towards Requirements for IP Routers, RFC 1716, P. Almquist, November 1994
4. ^ Requirements for IP Version 4 Routers, RFC 1812, F. Baker,June 1995

Further reading

• Chapter "Routing Basics" in the Cisco "Internetworking Technology Handbook"