Further to my post from a few months ago on the this topic (dating back to the RC build), I’ve seen this same problem a few more times on the RTM build of Windows Server 2008 R2. My suggested approach still fixes the problem and it doesn’t recur, but I’ve still not been able to pin down a cause and I can find no documentation on host machine MAC assignments anywhere. Search engines results are entirely focused on MAC pool duplication, which should be a completely distinct issue since the MAC address in question falls outside of Hyper-V’s MAC pool.
To help pin this down, I will add that this Hyper-V host machine has been deployed from a WDS image, but I initially discounted that as an issue because the Hyper-V role is not part of the image; it is added during the deployment as specified in an unattended installation file and the Hyper-v networks are created by script post-deployment. Since the Hyper-V host’s MAC address is distinct from the physical adapter’s address, I’ve been assuming that there must be an algorithm that generates the host’s virtual MAC address uniquely, but given that this problem seems to manifest itself soon after installation I’m leaving this possibility on the table.
I’ve noticed a fair amount of traffic for this topic. If you’re having the same problem and this suggestion fixes it, I’d be grateful to know if the Hyper-V system was deployed from an image, as that might help to narrow the troubleshooting effort.