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:
Many routing protocols are defined in documents called RFCs.[1][2][3][4]
The specific characteristics of routing protocols include
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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:
Interior Gateway Protocols (IGPs) exchange routing information within a single routing domain. A given autonomous system [5] can contain multiple routing domains, or a set of routing domains can be coordinated without being an Internet-participating autonomous system. Common examples include:
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.
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