3Com CoreBuilder 3500, Руководство по внедрению

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Areas 373
Types of Areas All routers within the same area maintain and use identical link state
advertisement (LSA) databases. The network shown in Figure 70 later in
this chapter contains four OSPF areas within autonomous system A. There
are three types of OSPF areas:
■
Transit area — An area through which network traffic can pass to
reach other areas in the autonomous system. In Figure 70, the
backbone area and areas 1 and 3 are transit areas.
■
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.
■
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 70,
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.