Packet Guide to Routing and Switching

Chapter 6. Open Shortest Path First

Open Shortest Path First (OSPF) and the Routing Information Protocol (RIP) are both interior routing protocols designed for networks within a single autonomous system, but the similarities end there. OSPF uses a completely different algorithm and is a link state protocol while RIP is a distance vector protocol. In the last chapter, RIP was shown to be a simple but reliable protocol. However, the limitations regarding overall network size and its slow convergence speed have restricted RIP deployments. When network topologies grow beyond fifteen hops or the topologies become more complex, protocols like OSPF not only become more attractive but a requirement.

This chapter will continue the tradition of building the sample topologies on Cisco equipment, exploring the ideas, packets and operation of OSPF. Like most protocols, getting OSPF up and running requires a small number of commands. However, OSPF can easily grow to significant complexity. OSPF has a lengthy specification and a number of RFCs that expand the original protocol. To keep the chapter readable, the commonly deployed features will be discussed here.

Protocol Description

The Open Shortest Path First specification was first described in the 1989 RFC 1131 but was quickly surpassed by RFC 1247 two years later which covered OSPF version 2. This version of the protocol has also been updated several times. For the purposes of this chapter, RFC 2328 (and packet captures) will form the basis of ...