Peering Policies and Route Servers

There is one thing that the RS can't do.

If A & B are doing 3rd party peering via the RS, the fact that the
A/RS peering is up & working and that the B/RS peering is up &
working unfortunately does not tell you if A & B can exchange

If A & B are peering directly, then the fact that the peering is
up also tells you that they can exchange packets.

Luckily this sort of breakage does not happen very often.
Unluckily, if it does break, if can be really hard to diagnose. (Andrew Partan)

It has happened at least a few times in the past month at MAE-East (islands of
disconnectivity), of course.