When best-external is enabled for an address family, all routes belonging to that address family must be classified internally as either ‟internal” or ‟external”. A route is ‟internal” if:
It was received from an iBGP peer in the same AS.
It was originated by a router in the same or a different RR cluster of the same AS.
A route is ‟external” if it was received from an eBGP peer in a different AS.
The tie-breaking steps of the decision process are run as usual on all of the routes (both ‟internal” and ‟external”) for a particular destination until only one path, the best path, is left. If this is an external route then the decision process must be rerun on only the ‟internal” routes to find the single best path in that subset. This ‟best internal” route is advertised to confed-eBGP peers, as described in Advertisement rules with best external.
If the overall best path found by the first run of the decision process is an internal route with NEXT_HOP n the decision process must be rerun on only the ‟external” routes with NEXT_HOP not equal to n to find the single best path in that subset. This ‟best external” route is advertised to iBGP peers, as described in Advertisement rules with best external.