OSPF areas

OSPF Areas

OSPF areas are used to impose a hierarchial structure to the flow of data over the network. A network using OSPF will always have at least one area and if there is more than one area, one of the two areas must be the backbone area. OSPF has only 2 levels to its hierarchy, the backbone, and all other areas attached to it. Areas are used to group routers into manageable groups that exchange routing information locally, but summarize that routing information when advertising the routes externally. A standard OSPF network looks something like a big bubble (the backbone area) with a lot of smaller bubbles (stub areas) attached directly to it. Area Border Routers (ABR) are used to connect the areas. Each area will elect a designated router (DR) and a backup designated router (BDR) to assist in flooding Link State Advertisements (LSAs)throughout the area.

Backbone (Area 0)

The backbone is the first area you should always build in any network using OSPF and the backbone is always Area 0 (zero). All areas are connected directly to the OSPF backbone area. When designing an OSPF backbone area, you should make sure there is little or no possibility of the backbone area being split into two or more parts by a router or link failure. If the OSPF backbone is split due to hardware failures or access lists, sizeable areas of the network will become unreachable.

Totally Stub Area

A totally stubby area is only connected to the backbone area. A totally stubby / totally stub area does not advertise the routes it knows. It does not send any Link State Advertisements. The only route a totally stub area receives is the default route from an external area, which must be the backbone area. This default route allows the totally stub area to communicate with the rest of the network.

Stub Area

Stub areas are connected only to the backbone area. Stub areas do not receive routes from outside the autonomous system, but do receive the routes from within the autonomous system, even if the route comes from another area.

Not-So-Stubby (NSSA)

Frequently, it is advisable to use a separate network to connect the internal enterprise network to the Internet. OSPF makes provisions for placing an Autonomous System Boundary Router (ASBR) within a non-backbone area. In this case, the stub area must learn routes from outside the OSPF autonomous system. Thus, a new type of LSA was required--the Type 7 LSA. Type 7 LSA's are created by the Autonomous System Boundary Router and forwarded via the stub area's border router (ABR) to the backbone. This allows the other areas to learn routes that are external to the OSPF routing domain.