I'm configuring a Hyper-V server (2008 R2) with two NICs. I'm trying to follow the MS best practice advice of leaving one NIC for the host OS. NIC #1 is used for the OS and has no associated virtual network. NIC #2 has a virtual network with "Allow management operating system to share this network adapter" unchecked. Both NICs are attached to the same physical switch.

From other PCs in the network, I can reach both the host and the guests without issue. However, between the host and the guests there is no connectivity.

Where should I begin to troubleshoot this?


I've since managed to solve the problem. I doubt that anyone else is dumb enough to make the same mistake, but I'll leave the answer here where it might at least give someone a good laugh.

Firstly, the "without issue" above wasn't entirely true - there were network dropouts and near-dropouts every now and then, Remote Desktop would stall, Hyper-V migrations would fail halfway, etc. I started digging through the system log and I noticed a lot of warnings about MAC addresses failing to register as they were already being used for Hyper-V.

I then remembered that I'd cannibalized one of the network cards in the host from an old server that was now running as a VM. I checked and sure enough, the P2V process had kept the old MAC address. For good measure, the MAC of the other gbit NIC in the box was used for another VM. So both of the NICs had MAC address conflicts with VMs running on the same machine...

I changed the MAC addresses of both affected VMs to "dynamic" and everything is working perfectly.

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