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

Chapter 133
Information from Other Autonomous Systems

Now, what about talking to other autonomous systems (outside of the OSPF domain)? Through the use of a special router type—the Autonomous System Boundary Router (ASBR)—OSPF networks can communicate with other ASs. This adds another level of hierarchy to the OSPF routing. The first is the intra-area routing. The second level is area-to-area routing through the backbone. The third level is external autonomous systems.

ASBRs run the OSPF protocol and some type of Exterior Gateway Protocol (such as Border Gateway Protocol defined in RFC 1403, BGP, or even RIP). RIP is seen as an external network and its routes are imported into a Link State Database as such. An external AS need not be another AS in the sense of a BGP. OSPF treats any routing protocol unlike itself as an external AS. This type of protocol allows for information to be exchanged between ASs. The EGP type of protocol only runs on the interfaces that are between the ASs. OSPF runs on the interfaces internal to the AS. An ASBR does not have to directly attach to the backbone.

To allow for this, another type of advertisement is used, known as the External Links Advertisement. Each ASBR in the AS generates one of the advertisements. This is the only advertisement that is flooded into every area in the AS. These advertisements describe routes that are external to the AS. There is one entry for every external route. As you can see, this could quickly fill up a routing table with external routes.

The external route uses one of two available types of metrics: Type 1 or Type 2. Type 1 metrics are the preferred route and are used when considering the internal cost of the AS. This means that Type 1 metrics include the Link State Metric as well as the metric that was assigned to it. Therefore, any router that receives this type of update for an external route must use the internal (AS) metrics to reach the ASBR advertising that external route. So, the computation for cost to reach that route uses metrics that are internal to the AS and the AS that was supplied in the advertisement.

Type 2 metrics are the same metrics that were advertised by the ASBR. Internal AS metrics are not added to the ASBR metric for the route when computing a path (based on cost) for that external route. (that is, to reach the ASBR advertising that external route).