Beginning with Cisco IOS XE Release 3.4S, BGP Best External is extended to BGP Best External for Intercluster RRs. This feature
provides path diversity between RR clusters, providing best external functionality toward non-client iBGP peers. The feature
is also known as the “intercluster best external path.”
Best external path at an RR means the best path within the RR’s cluster. This path might also be referred to as the best internal
When an RR (RR1) chooses a non-client iBGP path (that is, a path learned from another RR, let’s say RR2) as its overall best,
with the BGP Best External for Intercluster RRs feature, RR1 will be able to advertise its best internal path to the non-client
iBGP peers. This will help RR2 to learn an additional path, providing a diverse path.
Best external functionality at RRs is only for non-client iBGP peers. An RR cannot advertise best external paths to its clients
because it has to advertise its overall bestpath (which can be either a client path or non-client or eBGP path).
The best external path calculated by the RR is the best internal path for the cluster. It will be advertised to the non-client
iBGP peers only when the overall best path at this RR is a non-client iBGP path.
When there are multiple RRs, each in its own cluster, each RR must have the
neighbor advertise best-external
command configured for each of its neighbor RRs.
If the RR is in the forwarding plane, the
bgp additional paths install command is necessary.