This may be a dumb question, but dvSwitches make me nervous. In my test vCenter 5.5 environment, when I upgrade a esxi 5.1 to 5.5, I had to upgrade something related to dvSwitches on the host itself. I'm not sure if this caused any disruption or not since I didn't have any VM's on this host.
Assuming for now, vCenter is upgraded to 5.5 and my esxi hosts are still 5.1, will I run into any downtime as vCenter upgrades?