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

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.

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.

Important This section, method, or task 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. For more information about how to back up and restore the registry, click the following article number to view the article in the Microsoft Knowledge Base:

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: 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 name	UnicastInterHostCommSupport
   Value	1

   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

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).

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