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

Chapter 132
Inter-Area Routing

Routing a packet between areas involves transmitting a packet from its source, through its internal area to the ABR. The ABR transmits the packet over the backbone area to another ABR, where it is transmitted internally on the area to the destination host. Areas cannot route directly to other areas!

Again, the backbone area is a special area. Its main function is to distribute information from areas to other areas. It consists of networks that are not contained in any other defined area, and routers that are attached to an area or areas.

Why do areas? Breaking the AS into routable areas greatly reduces the amount of overhead in the form of routing information updates that need to be distributed throughout the AS. While this may not seem like much, remember, that each area can be unique. One area can have a majority of WAN links, while others are mostly networks, and others are a combination of multiple network types. Why make the update process very complex, and why bother other areas with your information? Remember, when the routing algorithm runs, every router in an area must run it. If one router in an area runs the algorithm, the routers in other areas may not have to run it. Dykstra runs in one area only. ABR will have a minimum of two copies of the Dykstra, one for each area it connects to.

Inter-Area Routing