Chapter 8. Understanding Open Shortest Path First (OSPF)

Chapter 8. Understanding Open Shortest Path First (OSPF)

This chapter covers the following key topics about the Open Shortest Path First (OSPF) protocol:

• OSPF packet details

• OSPF LSA details

• OSPF areas

• OSPF media types

• OSPF adjacencies

OSPF is a link-state interior gateway protocol designed for a large complex network. An IETF standard, OSPF is widely deployed in many large networks. Development began in 1987, and OSPF Version 2 was established in 1991 with RFC 1247. The goal was to have a link-state protocol that is more efficient and scalable than RIP. RFC 2328 (April 1998) is the latest revision to OSPF Version 2.

OSPF runs on top of IP and uses protocol number 89, just as TCP runs on top of IP and uses protocol number 6. OSPF doesn't use any transport protocol, such as TCP, for reliability. The protocol itself has a reliable mechanism of transportation.

OSPF is a classless routing protocol that supports variable-length subnet masking (VLSM) and discontiguous networks. OSPF employs multicast addresses 224.0.0.5 (all SPF routers) and 224.0.0.6 (designated routers [DR] and backup designated routers [BDR]) to send Hellos and updates. OSPF also provides two types of authentication ‚ plain text and message digest algorithm 5 (MD5).

OSPF uses the Dijkstra algorithm as a part of the routing table calculation process. The Dijkstra algorithm produces the shortest-path tree (SPT). Each router represents itself and its links to the neighbors in an understandable form ‚ link-state advertisements (LSAs). Based on information from the shortest path tree, OSPF can draw the network topology.

Each router in OSPF exchanges information about its cost, type of link, and network information with the other routers. Discussed later in the chapter, this multistep process is called link-state advertisement (LSA) exchange.