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
Read more about this topic: Routing Protocol