Regarding the mix of EX4600 and EX4300 in a VC, NSSU will be performed.
The advantage of preprovisioned mode is that you can control which devices can become RE/BK.
Not using preprovisioned mode can lead to unexpected results in term which FPCs are the RE/BK and their assigned FPC numbers.
Yes, you are right that if the two FPC acting as RE/BK in a preprovisoned mode fails, you will have a headless VC. This can be solved by letting any other FPC become one of the RE/BK.
However, it's unlikely that from a stack, you will have outage only for RE/BK. If those two fails in the same time, then probably the outage is big and probably the other FPCs went down.
On the other note, if only the RE goes down, it is expected to quickly check what happened with it and replace it. Nobody should run with a single RE.
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