A.1 Overview

By definition, all transit AS's must be able to carry traffic which originates from and/or is destined to locations outside of that AS. This requires a certain degree of interaction and coordination between BGP and the Interior Gateway Protocol (IGP) used by that particular AS. In general, traffic originating outside of a given AS is going to pass through both interior gateways (gateways that support the IGP only) and border gateways (gateways that support both the IGP and BGP). All interior gateways receive information about external routes from one or more of the border gateways of the AS via the IGP.

Depending on the mechanism used to propagate BGP information within a given AS, special care must be taken to ensure consistency between BGP and the IGP, since changes in state are likely to propagate at different rates across the AS. There may be a time window between the moment when some border gateway (A) receives new BGP routing information which was originated from another border gateway (B) within the same AS, and the moment the IGP within this AS is capable of routing transit traffic to that border gateway (B). During that time window, either incorrect routing or "black holes" can occur.

In order to minimize such routing problems, border gateway (A) should not advertise to any of its external peers a route to some set of exterior destinations associated with a given address prefix X via border gateway (B) until all the interior gateways within the AS are ready to route traffic destined to these destinations via the correct exit border gateway (B). In other words, interior routing should converge on the proper exit gateway before/advertising routes via that exit gateway to external peers.