Any OSPF (Open Shortest Path First) network that contains more than one area is required to have an area configured as area 0.0.0.0, also called the backbone. All areas in an AS must be connected to the backbone. When designing networks, you should start with area 0.0.0.0 and then expand into other areas.
Note
Area 0.0.0.0 exists by default and cannot be deleted or changed.The backbone allows summary information to be exchanged between area border routers (ABRs). Every ABR hears the area summaries from all other ABRs. The ABR then forms a picture of the distance to all networks outside of its area by examining the collected advertisements and adding in the backbone distance to each advertising router.
When a VLAN (Virtual LAN) is configured to run OSPF, you must configure the area for the VLAN.
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