28.1.1 Area border

The OSPFv3 protocol allows you to divide an autonomous system (AS) into a number of areas to minimize routing traffic and cut down the processing time required by the SPF algorithm.

The following figure shows an example of an OSPFv3 network topology in which an AS is divided into several areas.

Figure 28-1: Example of a OSPFv3 network topology using area-partitioning

A router that attaches to multiple areas is called an area border router. For example, Router 2 and Router 5 in the figure above are area border routers.

Information about the connection status of a given area is not reported outside that area. A router contains no information about the connection status of areas to which it is not connected.

<Structure of this section>

(1) Backbone

An area with an area ID of 0.0.0.0 is called backbone. The backbone has a special role in a split-area topology. If an AS is divided into areas, one of the areas must be designated as the backbone area. Be careful to avoid creating a configuration in which an AS has multiple backbones. In such a configuration, information about inter-area routes is spread across multiple backbones. This can result in unreachable routes being generated or the protocol failing to select the best route.

An area border router uses the backbone to disseminate routing information to other areas. For this reason, an area border router must connect to the backbone area.

(2) Notes on area division

Although dividing an AS into areas reduces the load on the routers and minimizes routing traffic, it also adds a level of complexity to the OSPFv3 algorithm. In particular, you can run into difficulties putting the appropriate fault handling in place. We recommend that you do not use area division unless you have a specific need to reduce the load on routers or networks.

(3) Notes on Area Border Router

An area border router runs an iteration of the SPF algorithm for each area it serves. It provides functionality that summarizes the topology information for the areas to which it is connected for distribution to other areas in the AS. Thus, an area border router that is associated with a large number of areas is subject to heavy loads. We recommend a network configuration that limits the number of areas each area border router serves.
When an area has only one area border router, a fault in that router can partition the area from the backbone, resulting in a loss of connectivity with other areas. When building a network, we recommend that you place more than one area border router in areas with servers and AS boundary routers that support mission-critical functions and connections, so that there are sufficient alternate routes available if a router fails.