Configuring OSPF Areas

OSPF allows collections of contiguous networks and hosts to be grouped together. Such a group, together with the routers having interfaces to any one of the included networks, is called an area. Each area runs a separate copy of the basic link-state routing algorithm. This means that each area has its own link-state database.

The topology of an area is invisible from the outside of the area, and routers internal to a given area know nothing of the detailed topology external to the area. This isolation of area detail enables the protocol to effect a marked reduction in routing traffic as compared to treating the entire Autonomous System as a single link-state domain. A router has a separate link-state database for each area it is connected to. Routers connected to multiple areas are called Area Border Routers (ABR). Two routers belonging to the same area have, for that area, identical area link-state databases.

An autonomous system can have one or more areas. A multiple area AS must define one of the areas as the backbone with an area ID of 0. Area IDs are assigned during network configuration using the network command (see Configuring Networks). All non-backbone areas in a multiple area AS must either be contiguous to the backbone or connected using a virtual-link. The backbone is responsible for distributing routing information between non-backbone areas. The backbone must be contiguous. However, it need not be physically contiguous; backbone connectivity can be established and maintained through the configuration of virtual links.

Virtual links can be configured between any two backbone routers that have an interface to a common non-backbone area. Such virtual links belong to the backbone. The protocol treats two routers joined by a virtual link as if they were connected by an unnumbered point-to-point backbone network.

See RFC 2328, OSPF Version 2, for further details on inter-area connectivity.

An Area ID can be any value from 0 - 4294967295, but is converted into the 32-bit dotted-quad format (area 50 would be displayed as 0.0.0.50; area 3546 would be displayed as 0.0.13.218)