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

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:
     HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\WLBS\Parameters
  3. Verify that the following information is present:
     MaskSourceMAC
     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.