The overall Internet topology may be viewed as an arbitrary interconnection of transit, multihomed, and stub AS's. In order to minimize the impact on the current Internet infrastructure, stub and multihomed AS's need not use BGP. These AS's may run other protocols (e.g., EGP) to exchange reachability information with transit AS's. Transit AS's using BGP will tag this information as having been learned by some method other than BGP. The fact that BGP need not run on stub or multihomed AS's has no negative impact on the overall quality of inter-AS routing for traffic that either destined to or originated from the stub or multihomed AS's in question.
However, it is recommended that BGP be used for stub and multihomed AS's as well. In these situations, BGP will provide an advantage in bandwidth and performance over some of the currently used protocols (such as EGP). In addition, this would reduce the need for the use of default routes and in better choices of Inter-AS routes for multihomed AS's.