Chapter 10. Routing Protocols

A routing protocol is a means whereby devices exchange information about the state of the network. The information collected from other devices is used to make decisions about the best path for packets to flow to each destination network.

Routing protocols are both protocols and applications. The protocols themselves sit at Layer 3 in the OSI model, while the applications that make the routing decision run at Layer 7. Many routing protocols exist, though only a few are in common use today. Older protocols are rarely used, though some networks may contain legacy devices that support only those protocols. Some firewalls and servers may support a limited scope of routing protocols—most commonly Routing Information Protocol (RIP) and Open Shortest Path First (OSPF)—but for the sake of simplicity, I will refer to all devices that participate in a routing protocol as routers.