4.5 Intra-domain protocol considerations

While no changes need be made to internal routing protocols to support the advertisement of aggregated routing information between autonomous systems, it is often the case that external routing information is propagated within interior protocols for policy reasons or to aid in the propagation of information through a transit network. At the point when aggregated routing information starts to appear in the new exterior protocols, this practice of importing external information will have to be modified. A transit network which imports external information will have to do one of:

1. use an interior protocol which supports aggregated routing

2. find some other method of propagating external information which does not involve flooding it through the interior protocol (i.e., by the use of internal BGP, for example).

3. stop the importation of external information and flood a "default" route through the internal protocol for discovery of paths to external destinations.

For case (a), the modifications necessary to a routing protocol to allow it to support aggregated information may not be simple. For protocols such as OSPF and IS-IS, which represent routing information as either a destination+mask (OSPF) or as a prefix+prefix-length (IS-IS) changes to support aggregated information are conceptually fairly simple; for protocols which are dependent on the class-A/B/C nature of networks or which support only fixed-sized subnets, the changes are of a more fundamental nature. Even in the "conceptually simple" cases of OSPF and IS-IS, an implementation may need to be modified to support supernets in the database or in the forwarding table.