3Com 4007R Implementation Guide - Page 462

462 CHAPTER 19: OPEN SHORTEST PATH FIRST (OSPF) ROUTING Types of Areas All routers within the same area maintain and use identical link state advertisement (LSA) databases. The network shown in Figure 54 later in this chapter contains four OSPF areas within autonomous system A. There are three types of OSPF areas: s Transit area - An area through which network traffic can pass to reach other areas in the autonomous system. In Figure 54, the backbone area and areas 1 and 3 are transit areas. s Backbone area - A contiguous area within an autonomous system that is divided into more than one area. The system defines the backbone area as 0.0.0.0. The backbone area distributes routing data between other areas in the autonomous system. By definition, the backbone area is also a transit area. s Stub area - Generally, an area with only one entry or exit router. As a result, external routes are never flooded into stub areas. Instead, the area border router that is attached to the stub area advertises a single default external route into the area. This relationship conserves significant LSA database space that would otherwise be used to store external link state advertisements flooded into the area. In Figure 54, area 2 is a stub area that is reached only through area border router 1. It is possible to have a stub area with multiple area border routers and multiple exit points. However, all of the exit points and routers must contain the same external routing data so that the choice of an exit point does not need to be made for each external destination. An area's network topology is visible only to systems inside that area; it is not visible to systems outside that area. Conversely, the systems within an area cannot see detailed network structures outside the area. Because of this restriction of topological information, you can control traffic flow between areas and reduce routing traffic to below the levels that occur when the entire autonomous system is a single routing domain.