NETWORKING: OSPF AREA 0

OSPF Area 0 Concept

OSPF designed with special restrictions when multiple areas are involved. If more than one area is configured in network, one of these areas has be to be area 0. This is called the backbone area. When designing networks it is best practice to start with area 0 and then extend into other areas as you move on.
The backbone area has to be at the center of all other areas, The requirement is all areas have to be physically connected to the backbone. The reasoning behind this is that OSPF expects all areas to inject routing information into the backbone area and in return the backbone will spread that information into other areas. The following diagram will provide more understanding about the flow of information in an OSPF network:

In the above diagram, all areas are directly connected to the backbone. In the rare situations where a new area is introduced that cannot have a direct physical access to the backbone, a virtual link will have to be configured.

Virtual links will be discussed later on. Different types of routing information which are coming from different types of areas. Routes that are generated from within an area are called intra-area routes. These routes are represented by the letter O in the IP routing table.

Routes that originate from other areas are called inter-area. The notation for these routes is O IA in the IP routing table.

Routes that originate from other routing protocols when redistribution happens and that are injected into OSPF called external routes. These routes are represented by O E2 or O E1 in the IP routing table.

Multiple routes to the same destination are preferred in the following order: intra-area, inter-area, external E1, external E2.