Link-State Routing Protocols
Also known as shortest path first (SPF) protocols, link-state routing protocols learn about every router in an AS and flood information about the state of their directly connected links to each router in the AS. A link is an interface on the router. The link's state is a description of that interface. Link-state protocols use this information to build a link-state database that gives the router a picture of the network. Link-state protocols have fast convergence because of the link-state database. On the other hand, they require more CPU power and memory. OSPF is a very scalable link-state protocol. It is the interior routing protocol of choice in large internetworks. OSPF is scalable because larger networks can be divided into areas. Generally, each router in an area is concerned with the link state of the other routers in its area; it does not store the link states of routers in different areas in its link-state database, thus conserving resources. OSPF is considered a classless routing protocol; it sends information about the interface's subnet mask in its updates. The ICND and CCNA exams cover OSPF in a single area. OSPF in multiple areas is covered on the CCNP exams.