Unicast NLB nodes cannot communicate over an NLB-enabled network adaptor in Windows Server 2003 (898867)


The information in this article applies to:

  • Microsoft Windows Server 2003, Web Edition
  • Microsoft Windows Server 2003, Standard Edition
  • Microsoft Windows Server 2003, Enterprise Edition
  • Microsoft Windows Server 2003, 64-Bit Enterprise Edition
Important This article contains information about how to modify the registry. Make sure to back up the registry before you modify it. Make sure that you know how to restore the registry if a problem occurs. For more information about how to back up, restore, and modify the registry, click the following article number to view the article in the Microsoft Knowledge Base:

256986 Description of the Microsoft Windows registry

SYMPTOMS

When you try to connect to a Network Load Balancing (NLB) cluster by using NLB Manager, you may not be able to connect to more than one node. You may also notice the following symptoms:
  • You cannot successfully use the ping command.
  • You cannot establish network connectivity between the nodes of an NLB cluster. This symptom occurs when the following conditions are true:
    • The nodes operate in unicast mode.
    • The nodes use a single network adaptor.

CAUSE

In Microsoft Windows NT Load Balancing Service (WLBS), in Microsoft Windows 2000 NLB, and in Microsoft Windows Server 2003 NLB, network load-balanced hosts that operate in unicast mode cannot communicate with each other. This behavior occurs because NLB makes the Media Access Control (MAC) address the same on all systems. Therefore, the network redirector never actually sends any packets to the other NLB hosts.

RESOLUTION

Warning Serious problems might occur if you modify the registry incorrectly by using Registry Editor or by using another method. These problems might require that you reinstall your operating system. Microsoft cannot guarantee that these problems can be solved. Modify the registry at your own risk.

Service pack information

To resolve this problem, obtain the latest service pack for Windows Server 2003. For more information, click the following article number to view the article in the Microsoft Knowledge Base:

889100 How to obtain the latest service pack for Windows Server 2003

After you apply the service pack, you must add a registry key on each system. To do this, follow these steps on the instance of NLB for which you want to enable this feature:
  1. Click Start, click Run, type regedit, and then click OK.
  2. Locate and then click the following registry subkey:

    HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\WLBS\Parameters\Interface\{GUID}

    Note The {GUID} placeholder represents the GUID of the particular NLB instance. You can use the ClusterIPAddress subkey in this hive to identify different NLB clusters.
  3. On the Edit menu, click New, click DWord Value, and then add the following value data.
    Value nameUnicastInterHostCommSupport
    Value1
    Note If you set the UnicastInterHostCommSupport registry entry to any non-zero value, Unicast InterHost Communication Support will be enabled.
  4. Quit Registry Editor.
  5. Open a command prompt, and then type the following command:

    NLB RELOAD

STATUS

Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section. This problem was corrected in Windows Server 2003 Service Pack 1 (SP1).

MORE INFORMATION

For more information, click the following article number to view the article in the Microsoft Knowledge Base:

824684 Description of the standard terminology that is used to describe Microsoft software updates

Modification Type:MinorLast Reviewed:6/15/2005
Keywords:kbBug kbfix kbQFE kbWinServ2003preSP1fix KB898867 kbAudITPRO
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