Network connectivity is lost on Hyper-V VMs if VMQ feature is enabled on HOST network cards

Article ID: 2681638 - View products that this article applies to.
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SYMPTOMS

Consider the scenario:

  • A server running Windows Server 2008 R2 with Hyper-V installed or Microsoft Hyper-V Server 2008 R2.
  • Live migration of VMs would result in drop of network connections on Guest VM that use VLAN. Network is restored when migration is complete.
  • Issue occurs only if Virtual Machine Queue (VMQ) is enabled on the HOST network and disabled on virtual networks.

Note: If we disable VMQ on Host network, Live migration of Guest VMs is successful without Network drop.

CAUSE

NICs are introduced with new feature "VMQ- Virtual Machine Queues". Earlier Hyper-V used to create the queue and segregate the traffic between the VMs, however with VMQ enabled, this option is offloaded to NICs. Creating and sorting of queues are done by the NICs. 

Just enabling VMQ on NIC is not sufficient. VMQ does require some registry for VMSMP to understand the VMQ feature and support it.

RESOLUTION

Caution This section contains steps that tell you how to modify the registry. However, serious problems might occur if you modify the registry incorrectly. Therefore, make sure that you follow these steps carefully. For added protection, back up the registry before you modify it. Then, you can restore the registry if a problem occurs.

To resolve this particular issue, follow below steps to add registry subkeys on the host server.
  1. To open an elevated Command Prompt window, click Start, point to All Programs, click Accessories, right-click Command Prompt, and then click Run as administrator.
  2. Type regedit, and then press ENTER.
  3. In the Registry Editor, open the sub-key HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Class\{4D36E972-E325-11CE-BFC1-08002BE10318 and locate the sub-key for the network adapter you want to work with. Sub-keys are four numbers (for example 0003 and 0010). Make a note of it. You will need it later in this procedure.
  4. Return to the elevated command prompt window.
  5. At the command prompt, type the following commands based on the type of network adapter you are using. For each command, substitute the sub-key from earlier in this procedure for ID.
    1. For 1 GBPS network adapters, type 
      reg add HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Class\{4D36E972-E325-11CE-BFC1-08002BE10318\ID /v *MaxRssProcessors /t REG_DWORD /d 1 /f
      press ENTER
      reg add HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Class\{4D36E972-E325-11CE-BFC1-08002BE10318\ID /v *RssBaseProcNumber /t REG_DWORD /d 0 /f
      and then press ENTER.
    2. For 10 GBPS network adapters, type 
      reg add HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Class\{4D36E972-E325-11CE-BFC1-08002BE10318\ID /v *MaxRssProcessors /t REG_DWORD /d 3 /f
      press ENTER, 
      reg add HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Class\{4D36E972-E325-11CE-BFC1-08002BE10318\ID /v *RssBaseProcNumber /t REG_DWORD /d 0 /f
      and then press ENTER.
    3. Reboot the Host server for registry changes to take affect.

Important: If you are configuring more than one network adapter, each adapter should have a different value assigned to the *RssBaseProcNumber sub-key with sufficient difference so that there are no overlapping RSS processors.
For example, if Network Adapter A has a value of 0 assigned to *RssBaseProcNumber and a value of 3 assigned to *MaxRssProcessors, Network Adapter B should have an *RssBaseProcNumber of 4.

MORE INFORMATION

VMQ is a new feature and is usually used with Hyper-V Clusters. Below is the complete checklist for VMQ.
  1. Virtual Machine Queue (VMQ) will only work on Windows 7 and Windows Server 2008 R2 Guests.
  2. If you have enabled Virtual Machine Queue (VMQ), Virtual Machine Chimney (VMC) will not work. You have to keep VMC disabled.
  3. Verify Synthetic NICs are configured for VMs.

Refer following articles for a more details:
Note This is a "FAST PUBLISH" article created directly from within the Microsoft support organization. The information contained herein is provided as-is in response to emerging issues. As a result of the speed in making it available, the materials may include typographical errors and may be revised at any time without notice. See Terms of Use for other considerations.

Properties

Article ID: 2681638 - Last Review: February 28, 2012 - Revision: 3.0
APPLIES TO
  • Windows Server 2008 R2 Datacenter
  • Windows Server 2008 R2 Enterprise
  • Windows Server 2008 R2 Service Pack 1
  • Microsoft Hyper-V Server 2008 R2
Keywords: 
KB2681638

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