NEXT_HOP Reachability
The NEXT_HOP MUST be reachable for the BGP path to be valid.
Reachability should be provided by the IGP.
Other route characteristics also important for best path selection
Change in the reachability characteristics of the NEXT_HOP (availability, cost) may impair the ability to forward traffic and/or cause black holes or routing loops.
BGP depends on the underlying IGP to provide fast and consistent notification of any change