A.2.5 Other Cases

There may be AS's with IGPs which can neither carry BGP information nor tag exterior routes (e.g., RIP). In addition, encapsulation may be either infeasible or undesirable. In such situations, the following two rules must be observed:

1. Information received from an internal peer by a border gateway A declaring a destination to be unreachable must immediately be propagated to all of the external peers of A.

2. Information received from an internal peer by a border gateway A about a reachable destination X cannot be propagated to any of the external peers of A unless A has an IGP route to X and sufficient time has passed for the IGP routes to have converged.

The above rules present necessary (but not sufficient) conditions for propagating BGP routing information to other AS's. In contrast to tagged IGPs, these rules cannot ensure that interior routes to the proper exit gateways are in place before propagating the routes to other AS's.

If the convergence time of an IGP is less than some small value X, then the time window during which the IGP and BGP are unsynchronized is less than X as well, and the whole issue can be ignored at the cost of transient periods (of less than length X) of routing instability. A reasonable value for X is a matter for further study, but X should probably be less than one second.

If the convergence time of an IGP cannot be ignored, a different approach is needed. Mechanisms and techniques which might be appropriate in this situation are subjects for further study.