Guest ethernet bridging not working

Question

Hi, I have the following setup:
Host system:
Windows Server 2008 R2 x64, Hyper-V installed
Guest system:
Windows Server 2008 R2 x64
One virtual NIC
I want to install OpenVPN on the guest system. To do that, OpenVPN creates its own TAP adapter and requires a network bridge to an internet-facing NIC on the machine. This is what my network connections looks like:
The Network Bridge is just between the Hyper-V virtual "LAN Adapter" and the "OpenVPN Adapter". But with the bridge enabled, I have no network connectivity. That is, if I right-click and view "Status" of the bridge, it shows many packets sent,
but zero received. After some experimentation, I've found:
With both LAN and OpenVPN adapters added to the bridge, I have no network connectivity.
With just the LAN adapter added to the bridge and the OpenVPN adapter disabled, still no network connectivity.
If I remove the LAN Adapter from the bridge or if I delete the bridge, the LAN Adapter itself then correctly connects and I have network connectivity.
Does anybody have any idea what's going on?

Answer 1

I figured it out. Network bridges do not work by default in Hyper-V guests because the virtual NIC ("Microsoft Virtual Machine Bus Network Adapter") doesn't automatically enter promiscuous mode when a bridge is created.
To fix this, run the following command in an elevated command prompt:
netsh bridge set adapter {number} forcecompatmode=enable
Reference:
http://support.microsoft.com/kb/302348