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.
Routing protocols allow networks to be dynamic and resistant to failure. If all routes in a network were static, the only form of dynamic routing we would be able to employ would be the floating static route. A floating static route becomes active only if another static route is removed from the routing table. Here’s an example:
ip route 0.0.0.0 0.0.0.0 192.168.1.1 1 ip route 0.0.0.0 0.0.0.0 10.0.0.1 2
The primary default route points to 192.168.1.1 and has a metric of 1. The second default route points to 10.0.0.1 and has a metric of 2.
Routes with the best metrics are inserted into the ...