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372 C
HAPTER
14: O
PEN
S
HORTEST
P
ATH
F
IRST
(OSPF)
Areas
To reduce the amount of routing information that travels through a
network, and the corresponding size of the OSPF routers’ topology
databases, subdivide OSPF autonomous systems into
areas . Each area has
the following configurable parameters:
■
Area ID — A 32 bit number that identifies the area to the OSPF
autonomous system. The Area ID is specified in the form n.n.n.n,
where 0
≤
n
≤
255. Subdividing the autonomous system (AS) into
multiple areas requires the use of a backbone area, which connects all
other areas in the AS and is always assigned an area ID of 0.0.0.0.
Although an area ID has the same superficial form as an IP address,
the area ID address space is its own distinct address space.
■
Stub area — An OSPF area that does not accept or distribute external
address advertisements. Instead, the area border router generates a
default external route that is advertised into the stub area for
destinations outside the autonomous system. Use the stub area
designation to minimize topological data that is stored in the area’s
routers.
■
Range — An address that covers a range of subnetwork addresses. A
range address aggregates LSAs from all of its subnetwork addresses;
this aggregation is also known as
route summarization
. For more
information, see “Configuring Route Summarization in ABRs” later in
this chapter.
■
Default route metric — The network cost for an OSPF default route.
If a default route metric is defined, the router advertises itself as the
default router to the area. For more information, see “Default Route
Metric” later in this chapter.