The following are some examples based on Figure 4-1:
?– Router B is the backbone router. The backbone router provides connectivity between different
areas.
?– Routers C, D, and E are area border routers (ABR). ABRs attach to multiple areas, maintain
separate link-state databases for each area to which they are connected, and route traf?¬?c
destined for or arriving from other areas.
?– Routers F, G, and H are nonbackbone, internal routers. Nonbackbone, internal routers are
aware of the topology within their respective areas and maintain identical link-state databases
about the areas.
?– Depending on the con?¬?guration of the OSPF nonbackbone area (stub area, totally stubby area,
or NSSA) the ABR advertises a default route to the nonbackbone, internal, router. The
nonbackbone, internal router uses the default route to forward all interarea or interdomain
traf?¬?c to the ABR router.
?– Router A is the autonomous system boundary router (ASBR) that connects to an external
routing domain, or autonomous system.
?– Router I is a router that belongs to another routing domain, or autonomous system.
Establishing OSPF Neighbor Adjacencies
Neighbor OSPF routers must recognize each other on the network before they can share
information because OSPF routing depends on the status of the link between two routers.
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