OSPF introduces a two level hierarchy for improving scalability. It allows an As to be partitioned into several groups called areas, that are interconnected by a central backbone area as shown in the Fig.1.

An area is identified by a 32-bit number known as the area ID. The backbone area is identified with area ID 193.0 .0 .0 The information from other area is summarized by area border router that have connections to multiple areas.

OSPF uses four types of routers

1. An internal router is a router with all its links connected to the networks within the same area.
2. An area border router is a router that has its links connected to more than one area.
3. A backbone router is a router that has its links connected to the backbone.
4. An autonomous system boundary router (ASBR) is a router that has its links connected to another autonomous system.

As shown in the Fig.1 routers $R_{1}, R_{2}$ and $R_{7}$ are internal routers. Routers $R_{3}, R_{6}, R_{8}$ are area border routers. Routers. Routers $R_{3}, R_{4}, R_{5}, R_{6}, R_{8}$ are backbone routers. Router $R_{4}$ is an ASBR.

A hello protocol allows neighbours to be discovered automatically. Two routers are said to be neighbours if they have an interface to a common network. The OSPF protocol runs directly over IP, using IP protocol 89.

The OSPF operation consists of the following stages

1.OSPF send the Hello messages for discovering the neighbours and designated routers are elected in multiaccess networks.

2.Adjacencies are established and link state databases are synchronized.

3.Link state advertisement are exchanged by adjacent routers to allow topological databases to be maintained and to advertise inter area and inter AS routes. The routers use the information in the database to generate routing tables.