The OSPF networks in an AS are administratively grouped into areas. Each area within an AS operates like an independent network and has a unique 32-bit area ID, which functions like a network address. Within an area, the topology database contains only information about the area, LSAs are flooded only to nodes within the area, and routes are computed only within the area. Subnetworks are divided into other areas, which are connected to form the whole of the main network.
The central area of an AS, called the backbone area, has a special function and is always assigned the area ID 0.0.0.0. Area IDs are unique numeric identifiers, in dotted decimal notation, but they are not IP addresses. Area IDs need only be unique within an AS. All other networks or areas in the AS must be directly connected to the backbone area by a router that has interfaces in more than one area. These connecting routers are called area border routers (ABRs). Figure 54 shows an OSPF topology of three areas connected by two area border routers.
Figure 54: Multiarea OSPF Topology
Area border routers are responsible for sharing topology information between areas. They summarize the link-state records of each area and advertise destination address summaries to neighboring areas. The advertisements contain the ID of the area in which each destination lies, so that packets are routed to the appropriate area border router. For example, in the OSPF areas shown in Figure 54, packets sent from Router A to Router C are automatically routed through Area Border Router B.