Understanding OSPF Areas

In OSPF, a single AS can be divided into smaller groups called areas. This reduces the number of link-state advertisements and other OSPF overhead traffic sent on the network, and it reduces the size of the topological database that each router must maintain.

This section discusses the following topics:

Areas

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. Routers 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 routers within an area have identical topological databases.

Area Border Routers

Routers that belong to more than one area are called area border routers. They maintain a separate topological database for each area to which they are connected.

Backbone Areas

An OSPF backbone area consists of all networks in area ID 0.0.0.0, their attached routers, and all area border routers. The backbone itself does not have any area border routers. The backbone distributes routing information between areas. The backbone is simply another area, so the terminology and rules of areas apply: a router that is directly connected to the backbone is an internal router on the backbone, and the backbone’s topology is hidden from the other areas in the AS.

The routers that make up the backbone must be physically contiguous. If they are not, you must configure virtual links to create the appearance of backbone connectivity. You can create virtual links between any two area border routers that have an interface to a common nonbackbone area. OSPF treats two routers joined by a virtual link as if they were connected to an unnumbered point-to-point network.

AS Boundary Routers

Routers that exchange routing information with routers in other ASs are called AS boundary routers. They advertise externally learned routes throughout the AS. Any router in the AS—an internal router, an area border router, or a backbone router—can be an AS boundary router.

Every router within the AS knows the path to the AS boundary routers.

Stub Areas

Stub areas are areas through which or into which AS external advertisements are not flooded. You might want to create stub areas when much of the topological database consists of AS external advertisements. Doing so reduces the size of the topological databases and therefore the amount of memory required on the internal routers in the stub area.

When an area border router is configured for a stub area, the router automatically advertises a default route in place of the external routes that are not being advertised within the stub area so that routers in the stub area can reach destinations outside the area.

The following restrictions apply to stub areas: you cannot create a virtual link through a stub area, and a stub area cannot contain an AS boundary router.

Not-So-Stubby Areas

An OSPF stub area has no external routes in it, so you cannot redistribute from another protocol into a stub area. A not-so-stubby area (NSSA) allows external routes to be flooded within the area. These routes are then leaked into other areas. However, external routes from other areas still do not enter the NSSA.

Transit Areas

Transit areas are used to pass traffic from one adjacent area to the backbone (or to another area if the backbone is more than two hops away from an area). The traffic does not originate in, nor is it destined for, the transit area.