Recently while working with a customer that was migrating from Hyper-V 2008 R2 to Hyper-V 2012, I came across an issue where VMs on the original host were having odd intermittent network connectivity issues. As I dug into the problem, I realized that when I ran arp -a on any of the Hyper-V hosts, VMs on the original host were showing up in the ARP cache with a MAC address of the physical host, instead of their virtual MAC address.
As it turned out, it was the drivers for the Broadcom NICs in this particular system. Windows Server 2008 does not support NIC teaming natively, and instead relies on the 3rd party drivers to support teaming. Due to a bug in some versions – possibly all, I have not been able to confirm this as of yet – of the Broadcom drivers, Hyper-V 2008 hosts using a Broadcom teamed interface will advertise the hosts’s MAC address to the physical switch rather than the VM’s virtual MAC.
The workaround was to break the team, and only use one physical interface during the migration to 2012. This resolved the MAC issues, and since Windows/ / Hyper-V 2012 supports native NIC teaming, the issue did not occur on the new hosts.
Disclosure: The company I work for is partners with several vendors - http://www.latisys.com/partners/strategic_partnerships.html