Types of Areas

Routers using OSPF send packets called Link State Advertisements (LSA) to all routers in an area. Areas are smaller groups within the AS that you can design to limit the flooding of an LSA to all routers. LSAs do not leave the area from which they originated, thus increasing efficiency and saving network bandwidth.

You must specify at least one area in your OSPF network - the backbone area, which has the responsibility to propagate information between areas. The backbone area has the identifier 0.0.0.0.

You can designate other areas, depending on your network design, of the following types:

• Normal Area - Allows all LSAs to pass through. The backbone is always a normal area.
• Stub Area - Stub areas do not allow Type 5 LSAs to be propagated into or throughout the area and instead depends on default routing to external destinations. You can configure an area as a stub to reduce the number of entries in the routing table (routes external to the OSPF domain are not added to the routing table).
• NSSA (Not So Stubby Area) - Allows the import of external routes in a limited fashion using Type-7 LSAs. NSSA border routers translate selected Type 7 LSAs into Type 5 LSAs which can then be flooded to all Type-5 capable areas. Configure an area as an NSSA if you want to reduce the size of the routing table, but still want to allow routes that are redistributed to OSPF.

Best Practice - It is generally recommended that you limit OSPF areas to about 50 routers based on the limitations of OSPF (traffic overhead, table size, convergence, and so on).

All OSPF areas must be connected to the backbone area. If you have an area that is not connected to the backbone area, you can connect it by configuring a virtual link, enabling the backbone area to appear contiguous despite the physical reality.

Note - If you need to connect two networks that both already have backbone areas and you do not want to reconfigure one to something other than 0.0.0.0, you can connect the two backbone areas using a virtual link.

Each router records information about its interfaces when it initializes and builds an LSA packet. The LSA contains a list of all recently seen routers and their costs. The LSA is forwarded only within the area it originated in and is flooded to all other routers in the area. The information is stored in the link-state database, which is identical on all routers in the AS.