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

Chapter 130
The Area Border Router (ABR)

There is a special router type known as the Area Border Router. Its job is to connect an area to the backbone and to summarize its area topology information (for all areas that it connects to) to the backbone area, where it is received by other ABRs to be included in their tables. ABRs also receive other area summaries from the backbone and propagate this to their areas. ABRs are part of the backbone area; therefore, ABRs belong to a minimum of two areas: their own and the backbone area. If there is only one area in the AS (the backbone area), there are no ABRs.

Since an ABR belongs to two or more areas, it has a separate database for each area to which it belongs. It also executes a single copy of the routing algorithm for each area to which it belongs. For a typical ABR, it maintains connections to its area and to the backbone area. For its area, it receives flooded LSAs that are within its area and maintains a synchronized database for the area. The other copy of the algorithm runs for the attachment to the backbone. ABRs do not flood learned information about its area to the backbone. It summarizes this information using summary link advertisements. These advertisements are pushed to other ABRs on the backbone, allowing those areas to learn about each other without directly participating in the backbone’s routing advertisements (remember, the backbone is a real area, too!).

Since, area reachability information is propagated only over the backbone area, every area must touch the backbone through the use of an ABR. An area is not allowed to be segmented from the backbone. A special condition does exist to allow an area to be extended off an area that is not the backbone through a concept known as a virtual link.