Illustrated TCP/IP by Matthew G. Naugle Wiley Computer Publishing, John Wiley & Sons, Inc. ISBN: 0471196568   Pub Date: 11/01/98

Chapter 128
OSPF Areas

OSPF uses a concept known as an area. Refer to Slide 140.0. An area is a grouping of contiguous networks and its associated routers that have interfaces belonging to those networks and hosts. An OSPF autonomous system can simply be one area (in this case, it must be the backbone area) or it can consist of many areas. Each area runs its own copy of the link-state routing algorithm, allowing for each area to build its own topological database. It is important to note that an area limits the flooding of an LSA. LSAs do not leave the area from which they originated. Furthermore, splitting a domain into areas allows for routing traffic bandwidth savings as well.

Each area is identified with a 32-bit number known as the area ID. This number is formatted in the same manner as an IP address; however, it has nothing to do with the IP addressing scheme of your network. It simply identifies an area. Common area IDs are 0.0.0.0 (a single area must be configured with the area ID of 0.0.0.0, or a multiple area must have one of its areas labeled 0.0.0.0, known as Area 0). Other area IDs are 1.1.1.1 to identify area 1, or 0.0.0.1 to identify area 1. There is no strict method to accomplish area ID numbering except for area 0.0.0.0.

Furthermore, each router in an area is assigned an router ID regardless of its area ID assignment . This number is a 32-bit number that uniquely identifies that router in the autonomous system. Typically, the router ID is chosen from the IP address assignments of one of the router interfaces.

The topology of an area in not known to any other area. This means that the internal routers of an area do not contain any information about the OSPF topology outside their area, giving the benefit of reduced routing overhead. A single area that is spread over sparse environments (WAN links) must contain the same topology database for the entire area no matter how large it has become. So how does an area determine routes that are not within its area? This is accomplished via the backbone area and the summary links advertisement.

ABRs play an important role in an OSPF network. Since areas do not know the topology in areas other than their own, some mechanism must be provided to allow network reachability information to traverse different areas. After all, it wouldn’t do much good to be able to dynamically route in your own area and then have to statically point to networks in other areas. ABRs compact the topological information for an area and transmit it to the backbone area. Routers in the backbone area make sure that it is forwarded to the areas that are attached to it. In order to accomplish this, ABRs run multiple copies of the OSPF algorithm, one for each area (including the backbone area). Areas also allow for the advantages of hierarchical topologies to be built.

OSPF Areas