Aldrin sent me an interesting question as a comment to one of my EVPN blog posts:

How does the network know that a VTEP is actually alive? (1) from the point of view of the control plane and (2) from the point of view of the data plane? And how do you ensure that control and data plane liveness monitoring has the same view? BFD for BGP is a possible solution for (1) but it’s not meant for 3rd party next hops, i.e. it doesn’t address (2).

Let’s stop right there (or you’ll stop reading in the next 10 milliseconds). I will also try to rephrase the question in more generic terms, hoping Aldrin won’t mind a slight detour… we’ll get back to the original question in another blog post.

Jeff Tantsura published a great response to my Can We Trust BGP Next Hops blog post on LinkedIn, and I asked him for permission to save it in a more permanent form. Here it is (slightly edited)…

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I’d like to bring back EVPN context. The discussion is more nuanced, the common non-arguable logic here - reachability != functionality.

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: