Tuesday, April 21, 2020

Can We Trust BGP Next Hops (Part 2)?

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)

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