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

• Microsoft Windows 2000 Advanced Server
• Microsoft Windows 2000 Datacenter Server
• Windows NT Load Balancing Service 1.0

SYMPTOMS

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.

CAUSE

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.

RESOLUTION

WARNING: Using Registry Editor incorrectly can cause serious problems that may require you to reinstall your operating system. Microsoft cannot guarantee that problems resulting from the incorrect use of Registry Editor can be solved. Use Registry Editor at your own risk.

For information about how to edit the registry, view the "Changing Keys and Values" Help topic in Registry Editor (Regedit.exe) or the "Add and Delete Information in the Registry" and "Edit Registry Data" Help topics in Regedt32.exe. Note that you should back up the registry before you edit it. If you are running Windows NT, you should also update your Emergency Repair Disk (ERD).

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.

Additional query words:

Keywords : kbenv kbnetwork
Version : WINDOWS:2000; winnt:1.0
Platform : WINDOWS winnt
Issue type : kbprb