Microsoft KB Archive/247297

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Network Load Balancing Connection to a Virtual IP Address Not Made Across a Switch

Article ID: 247297

Article Last Modified on 3/2/2007


  • Microsoft Windows 2000 Advanced Server
  • Microsoft Windows 2000 Datacenter Server
  • Microsoft Windows NT Load Balancing Service

This article was previously published under Q247297

IMPORTANT: This article contains information about modifying the registry. Before you modify the registry, make sure to back it up and make sure that you understand how to restore the registry if a problem occurs. For information about how to back up, restore, and edit the registry, click the following article number to view the article in the Microsoft Knowledge Base:

256986 Description of the Microsoft Windows Registry


When you attempt to establish a connection to the virtual IP address of a Network Load Balancing (NLB) cluster, the connection cannot be made across a switch. When the cluster is connected to a hub with a client, the client is able to make a connection without a problem.


This problem occurs because a large number of unknown unicast and multicast packets are being blocked on the port(s) of the switch to which the NLB virtual adapters are connected directly or uplinked through a hub. By default, NLB masks the source media access control (MAC) address on outbound packets, preventing switches from learning and forcing them to broadcast packets for unknown addresses to all ports.


WARNING: If you use Registry Editor incorrectly, you may cause serious problems that may require you to reinstall your operating system. Microsoft cannot guarantee that you can solve problems that result from using Registry Editor incorrectly. Use Registry Editor at your own risk.

Use one of the following methods to allow the virtual IP address to be reachable across the switch:

  • Disable port blocking (this allows unknown unicast and multicast packets to flood the specific ports).
  • Add a static address to the MAC address table on the switch that maps to the virtual MAC address used by the NLB cluster.
  • Make the following registry change on each NLB cluster host:
    1. Start Registry Editor (Regedt32.exe).
    2. Locate the following registry key:


    3. Verify that the following information is present:

      Data type = REG_DWORD
      Range = 0 or 1
      Default = 1

      Setting the Range value to 1 masks the source MAC address in outbound packets, preventing switches from learning and forcing them to broadcast packets for unknown addresses to all ports. Setting this value to 0 disables masking of the MAC address.

Keywords: kbenv kbnetwork kbprb KB247297