Routing protocols allow routers to dynamically share information about remote networks as shown in the figure. Routers receiving the update automatically add this information to their own routing tables. The routing protocols then determine the best path, or route, to each network. A primary benefit of dynamic routing protocols is that routers exchange routing information when there is a topology change. This exchange allows routers to automatically learn about new networks and also to find alternate paths when there is a link failure to a current network.
Compared to static routing, dynamic routing protocols require less administrative overhead. However, the expense of using dynamic routing protocols is dedicating part of a router’s resources for protocol operation, including CPU time and network link bandwidth. Despite the benefits of dynamic routing, static routing still has its place. There are times when static routing is more appropriate and other times when dynamic routing is the better choice. However, it is important to understand that static and dynamic routing are not mutually exclusive. Rather, most networks use a combination of dynamic routing protocols and static routes.
The two most common dynamic routing protocols are EIGRP and OSPF. The focus of this chapter is on OSPF.
Note: All dynamic routing protocols are capable of advertising and propagating static routes in their routing updates.