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The hierarchical topology possibilities of OSPF yield several important advantages:
Reduced frequency of SPF calculations
Smaller routing tables
Reduced Link State Updates overhead
Rather than send an LSU about each network to every area, a single route or a few summarized routes can be summarized between areas to reduce the overhead associated with LSUs that cross multiple areas.
Backbone area (Area 0) – A transit area, the central entity to which all other areas connect, all other areas must connect to this area to exchange route information. Accepts all LSAs .
Stub area - Area that does not accept information about routes external to the autonomous system (AS), such as routes from non-OSPF sources. If routers need to reach networks outside the AS, they use a default route. Does not accept external LSAs .
You can configure an OSPF router interface to either operate in a stub area (does not accept information about routes external to the AS (or Lsa-5)) or as a totally stubby area (does not accept LSA-5 and LSA-3) from other areas internal to the AS).
By configuring an area as stub, you can greatly reduce the size of the link-state database inside that area and, as a result, reduce the memory requirements of area routers .
If a new area is added after the OSPF internetwork has been designed, and it is not possible to provide that new area with direct access to the backbone, a virtual link can be defined to provide the needed connectivity to the backbone area.
Because all areas must be connected to Area 0, the virtual link provides the disconnected area a logical path to the backbone .
On each router that will use the virtual link, create the "virtual link" configuration. The routers that make the links are the ABR that connects the remote area to the transit area and the ABR that connects the transit area to the backbone area:
router(config-router)# area area-id virtual-link router-id