3com 2500 User Guide
Elements of OSPF Routing
7-5
Routing Databases
All routers connected to an area maintain identical routing databases
about the area. Routers connected to multiple areas maintain a separate
routing database for each attached area. For example, in Figure 7-1:
about the area. Routers connected to multiple areas maintain a separate
routing database for each attached area. For example, in Figure 7-1:
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Routers 1, 2, 3, and 4 maintain identical routing databases about
backbone area 0.
backbone area 0.
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Routers 5 and 6 maintain identical routing databases about area 1.
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Area border router 1 maintains separate routing databases about
backbone area 0 and area 2.
backbone area 0 and area 2.
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Area border router 2 maintains separate routing databases about
backbone area 0 and area 1.
backbone area 0 and area 1.
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Area border router 3 maintains separate routing databases about
backbone area 0 and area 3.
backbone area 0 and area 3.
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Area border router 4 maintains separate routing databases about
areas 1 and 3.
areas 1 and 3.
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Autonomous system boundary routers 1 and 2 maintain separate
routing databases about autonomous systems A and B.
routing databases about autonomous systems A and B.
In the CoreBuilder
™
2500 implementation of the OSPF model, each area
has the following configurable parameters:
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Area ID — The number, in the form of an IP address, that identifies
the area to the OSPF autonomous system
the area to the OSPF autonomous system
A backbone area must have the area ID 0.0.0.0. Routers in the backbone
area also must be able to communicate with each other through
interfaces that are configured as in area 0.
area also must be able to communicate with each other through
interfaces that are configured as in area 0.
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Stub area — An OSPF area that does not accept or distribute external
address advertisements. Use the stub area designation to minimize
topological data stored in the area’s routers.
address advertisements. Use the stub area designation to minimize
topological data stored in the area’s routers.
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Range — An address that covers a range of subnetwork addresses. A
range address aggregates LSAs from all of its subnetwork addresses.
range address aggregates LSAs from all of its subnetwork addresses.
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Default route metric — The network cost for an OSPF default route.
If the cost is greater than 0, the router advertises itself as the default
router to the area.
If the cost is greater than 0, the router advertises itself as the default
router to the area.
Default
The system default value for the default route metric is 0, which means
that the router does not advertise itself as the area’s default router.
that the router does not advertise itself as the area’s default router.