Configuring the ECN330-switch
442 1553-KDU 137 365 Uen D 2006-06-16
6.20.10.2 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, an area can be configured to export a single summarized route that
covers a broad range of network addresses within the area (page 446). To
further reduce the amount of routes passed between areas, 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.
Figure 196 OSPF Areas
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
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