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Configuring OSPF Areas
OSPF protocol broadcast messages (that is, Link State Advertisements or LSAs) are
restricted by area to limit their impact on network performance. A large network
should be split up into separate OSPF areas to increase network stability, and to
reduce protocol traffic by summarizing routing information into more compact
messages. Each router in an area shares the same view of the network topology,
including area links, route summaries for directly connected areas, and external
links to other areas.
Use the Area Configuration screen to define an OSPF area and the interfaces that
operate within this area. 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 20-23). 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.
area
area
stub
A
BR
ABR
ABR
default
external
route
backbone
backbone
By default, a stub can only pass traffic to other areas in the autonomous system via
the default external route. However, you also can configure an area border router to
send Type 3 summary link advertisements into the stub about subnetworks located
elsewhere in the autonomous system.
20-19
Configuring the Open Shortest Path First Protocol
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