An OSPF network can be divided into sub-domains called areas.In OSPF, a single autonomous system (AS) can be divided into smaller groups called areas.
An area is a logical collection of OSPF networks, routers, and links that have the same area identification. A router within an area must maintain a topological database for the area to which it belongs.
An area is a set of networks and hosts within an AS that have been administratively grouped together. We recommend that you configure an area as a collection of contiguous IP subnetted networks. Routing devices that are wholly within an area are called internal routers. All interfaces on internal routers are directly connected to networks within the area.
The topology of an area is hidden from the rest of the AS, thus significantly reducing routing traffic in the AS. Also, routing within the area is determined only by the area’s topology, providing the area with some protection from bad routing data.
All routing devices within an area have identical topology databases.
This reduces the number of link-state advertisements (LSAs) and other OSPF overhead traffic sent on the network, and it reduces the size of the topology database that each router must maintain. The routing devices that participate in OSPF routing perform one or more functions based on their location in the network.