Area Border Routers - 3Com CoreBuilder 2500 User Manual

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HAPTER OUTING WITH
OSPF
Backbone — A contiguous area within an autonomous system that is
divided into more than one area. The system automatically 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.
An area's network topology is not visible outside the area. Conversely, an
area's systems 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.

Area Border Routers

Each area (including the backbone area) includes all border routers
connected to the area. In Figure 7-1, for example, you define:
Area border routers 1, 2, and 3 as being in the backbone area 0
Area border routers 2 and 4 as being in area 1
Area border router 1 as being in area 2
Area border routers 3 and 4 as being in area 3
Routers must communicate with each other through interfaces that are
defined as being in the same area. An area, however, may contain virtual
links from area border routers to the backbone area. For example, in
Figure 7-1, area border routers 3 and 4 terminate a virtual link between
area 1 and the backbone area. See "Virtual Links" later in this chapter for
more details.
CAUTION: Do not disconnect an area border router from the backbone
area. This action may result in a loss of network topology information and
routing capability. You must connect all area border routers to the
backbone area using either physical or virtual links.