Enabling the best-external feature is supported only at the config>router>bgp level. This feature can be enabled/disabled on a per address family basis, with IPv4 and IPv6 as the only options supported initially. Enabling best-external for IPv4 causes the new advertisement rules to apply to both regular IPv4 unicast routes as well as labeled-IPv4 (SAFI4) routes. Similarly, enabling best-external for IPv6 causes the new advertisement rules to apply to both regular IPv6 unicast routes as well as labeled-IPv6 (SAFI4) routes.
The advertise-external command cannot be applied to a route reflector unless client-to-client reflection is disabled (disable-client-reflect in the CLI).