Best-external is a BGP enhancement that allows a BGP speaker to advertise to its IBGP peers its best ‟external” route for a prefix/NLRI when its best overall route for the prefix/NLRI is an ‟internal” route. This is not possible in a normal BGP configuration because the base BGP specification prevents a BGP speaker from advertising a non-best route for a destination.
In specific topologies best-external can improve convergence times, reduce route oscillation and allow better loadsharing. This is achieved because routers internal to the AS have knowledge of more exit paths from the AS. Enabling add-paths on border routers of the AS can achieve a similar result but add-paths introduces NLRI format changes that must be supported by BGP peers of the border router and therefore has more interoperability constraints than best-external (which requires no messaging changes).
Best-external is supported in the base router BGP context. (A related feature is also supported in VPRNs; consult the Services Guide for more details.) It is configured using the advertise-external command, which provides IPv4, label-IPv4, IPv6, and label-IPv6 as options.
The advertisement rules when advertise-external is enabled can be summarized as follows.
If a router has advertise-external enabled and its best overall route is a route from an IBGP peer then this best route is advertised to EBGP and confed-EBGP peers, and the ‟best external” route is advertised to IBGP peers. The ‟best external” route is the one found by running the BGP path selection algorithm on all LOC-RIB paths except for those learned from the IBGP peers.
If a router has advertise-external enabled and its best overall route is a route from an EBGP peer then this best route is advertised to EBGP, confed-EBGP, and IBGP peers.
If a router has advertise-external enabled and its best overall route is a route from a confed-EBGP peer in member AS X then this best route is advertised to EBGP, IBGP peers and confed-EBGP peers in all member AS except X and the ‟best external” route is advertised to confed-EBGP peers in member AS X. In this case the ‟best external” route is the one found by running the BGP path selection algorithm on all RIB-IN paths except for those learned from member AS X.