Two weeks ago I started with a seemingly simple question:
If a BGP speaker R is advertising a prefix A with next hop N, how does the network know that N is actually alive and can be used to reach A?
… and answered it for the case of directly-connected BGP neighbors (TL&DR: Hope for the best).
Jeff Tantsura provided an EVPN perspective, starting with “the common non-arguable logic is reachability != functionality".
Now let’s see what happens when we add route reflectors to the mix. Here’s a simple scenario:
Thanks to (see source)