3com 2500 User Guide

OSPF routing uses the following network elements:

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An autonomous system consists of a set of OSPF routers that exchange 
routing information. The network shown in Figure 7-1 contains two 
autonomous systems.

Using identical topology databases, each router in an autonomous system 
calculates shortest-path routes from itself to every known destination in 
the autonomous system. The routers create their topology databases 
using the data in link state advertisements (LSAs) from other routers in 
the autonomous system. 

You can reduce the amount of routing information that travels through a 
network, and the corresponding size of OSPF routers’ topology 
databases, by subdividing OSPF autonomous systems into areas. The 
routers in an area maintain and use identical LSA databases. 

The network shown in Figure 7-1 contains four OSPF areas within 
autonomous system A. There are three types of OSPF areas:

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Transit — An area through which network traffic can pass to reach 
other areas in the autonomous system. In Figure 7-1, the backbone 
area and areas 1 and 3 are transit areas.

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Stub — An area with only one entry or exit router. In Figure 7-1, 
area 2 is a stub area that is reached only through area border router 1.