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OUTING
3-309
Configuring OSPF Areas
An autonomous system must be configured with a backbone area,
designated by area identifier 0.0.0.0. By default, all other areas are created
as normal transit areas.
Routers in a normal area may import or export routing information about
individual nodes. To reduce the amount of routing traffic flooded onto the
network, you can configure an area to export a single summarized route
that covers a broad range of network addresses within the area
(page 3-313). To further reduce the amount of routes passed between
areas, you can configure an area as a stub or a not-so-stubby area (NSSA).
Normal Area – A large OSPF domain
should be broken up into several areas to
increase network stability and reduce the
amount of routing traffic required
through the use of route summaries that
aggregate a range of addresses into a
single route. The backbone or any
normal area can pass traffic between
other areas, and are therefore known as
transit areas. Each router in an area has identical routing tables. These
tables may include area links, summarized links, or external links that
depict the topology of the autonomous system.
Stub – A stub does not accept external
routing information. Instead, an area
border router adjacent to a stub can be
configured to send a default external
route into the stub for all destinations
outside the local area or the autonomous
system. This route will also be
advertised as a single entry point for traffic entering the stub. Using a stub
can significantly reduce the amount of topology data that has to be
exchanged over the network.
backbone
area
ABR
area
ABR
backbone
stub
ABR
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route