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Virtual Server 2008 NIC losing connectivity.

Last Modified: 2013-11-06
We are having an intermittent network connectivity problem on one of our Hyper-V virtual machines and need help.  The VM will run fine for weeks, and then without warning the virtual network card stops routing traffic.  In-bound and outbound connections are down when this happens, and the network icon on the task bar shows an exclamation indicating it is broken.  I’ve tried clicking on it and allowing it to attempt to repair, but it fails.  I’ve tried disabling the virtual adapter in the past so I could attempt to re-enable it, but it hangs and a reboot is required.  In fact, the only way to recover from this is to reboot the VM.  Once it comes back up, network operation is normal until the next time.
Here are the details of our environment:  We have a physical Windows 2008 Enterprise Server running as the Hyper-V host machine for 5 public virtual machines each with static public IP addresses (used for hosting public websites, email, etc). 4 of the VMs are running Windows 2008 server, and the 5th is running Windows 2003 server.  The physical Hyper-V host machine has two physical NIC cards, but only one is in use and plugged in to our network switch.  When I open the Network Connections window in the Control Panel, it shows three devices, the unplugged Local Area NIC card, the Enabled Local Area NIC card which is set up as a virtual switch using the Microsoft Virtual Network Switch Protocol, and a Virtual Local Area Connection.  The Virtual Network Connection is configured with a static unique IP address (IPv4).  The IPv6 box is checked also although it’s not used (maybe this is an issue?).  The Virtual Local Area Connection was created through the Hyper-V Virtual Network Manager, and in the properties the connection type is “External”, and the “Allow management OS to share this network adapter” is enabled.  Nothing else is checked.
All of our machines, both physical and virtual are on one Class C block of IP addresses sharing the same default gateway. The Windows 2008 Virtual Machine that occasionally loses connectivity has one Local Area Connection (Microsoft Virtual Machine Bus Network Adapter) with IPv4 static IP addresses configured.  I did notice that IPv6 is also checked even though it is not in use, could this possibly be part of the problem?   Any help in troubleshooting this issue would be greatly appreciated.
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