3.7. Partitions of areas

OSPF does not actively attempt to repair area partitions. When an area becomes partitioned, each component simply becomes a separate area. The backbone then performs routing between the new areas. Some destinations reachable via intra-area routing before the partition will now require inter-area routing.

In the previous section, an area was described as a list of address ranges. Any particular address range must still be completely contained in a single component of the area partition. This has to do with the way the area contents are summarized to the backbone. Also, the backbone itself must not partition. If it does, parts of the Autonomous System will become unreachable. Backbone partitions can be repaired by configuring virtual links (see Section 15).

Another way to think about area partitions is to look at the Autonomous System graph that was introduced in Section 2. Area IDs can be viewed as colors for the graph's edges.[1] Each edge of the graph connects to a network, or is itself a point-to- point network. In either case, the edge is colored with the network's Area ID.

A group of edges, all having the same color, and interconnected by vertices, represents an area. If the topology of the Autonomous System is intact, the graph will have several regions of color, each color being a distinct Area ID.

When the AS topology changes, one of the areas may become partitioned. The graph of the AS will then have multiple regions of the same color (Area ID). The routing in the Autonomous System will continue to function as long as these regions of same color are connected by the single backbone region.