A routing protocol 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.

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