7.3.2.2 Protocol Walk-through

While BGP provides support for quite complex routing policies (as an example see Section 4.2 in [ROUTE:5]), it is not required for all BGP implementors to support such policies. At a minimum, however, a BGP implementation:

1. SHOULD allow an AS to control announcements of the BGP learned routes to adjacent AS's. Implementations SHOULD support such control with at least the granularity of a single network. Implementations SHOULD also support such control with the granularity of an autonomous system, where the autonomous system may be either the autonomous system that originated the route, or the autonomous system that advertised the route to the local system (adjacent autonomous system).

2. SHOULD allow an AS to prefer a particular path to a destination (when more than one path is available). Such function SHOULD be implemented by allowing system administrator to assign weights to Autonomous Systems, and making route selection process to select a route with the lowest weight (where weight of a route is defined as a sum of weights of all AS's in the AS_PATH path attribute associated with that route).

3. SHOULD allow an AS to ignore routes with certain AS's in the AS_PATH path attribute. Such function can be implemented by using technique outlined in (2), and by assigning infinity as weights for such AS's. The route selection process must ignore routes that have weight equal to infinity.