Several months ago we got a call from a client with a standalone 2012 R2 HyperV host: Absolutely NO network traffic was going through any of the virtual switches, yet we could see no errors or warnings of any kind in any of the logs. Desperate to get them back up and running we simply deleted all the virtual switches, recreated them (identical to what they were) and reattached all the VMs. Note that the port reserved for the host had no issue. At first we chalked it up as a fluke....except that it happened again to a completely different customer. And then third. And then a forth. And then several months after the first incident it happened againt to the first customer. It has become a chronic issue which happens ever few months and no trigger can be found.
The only thing common to all of them is A) Server 2012 R2 host and B) HP ProLiant G8 servers (various models, inluding DL360, ML350, and ML310).
All hardware has the latest firmware and drivers as prescribed by HP. All Windows updates are applied in timely fashion. No event, change, or tampering precipitates the event. The virtual switches just stop passing data.
I've reached out to the Microsoft community on technet but no one there has seen this behavior before. I hope someone here can shed some light on it.