Microsoft KB Archive/258609

= WLBS cluster appears to stop servicing clients =

Article ID: 258609

Article Last Modified on 11/20/2006

-

APPLIES TO


 * Microsoft Windows NT Server 4.0 Standard Edition
 * Microsoft Windows NT Server 4.0 Enterprise Edition

-



This article was previously published under Q258609



SYMPTOMS
Host members of a Windows Load Balancing Service (WLBS) cluster that is configured to operate in Unicast mode (Multicast support is disabled) and is directly connected to a switch may suddenly stop servicing clients. Rebooting the cluster hosts appears to correct the problem.



CAUSE
By default, WLBS masks the cluster's virtual media access control (VMAC) address when sending packets to prevent a switch from learning the VMAC address and associating it with a port (for WLBS to work properly, packets going to the virtual IP address must reach all cluster hosts). If the switch associates the VMAC address with a port, packets going to the virtual IP address are sent to just the cluster host that is connected to the port on which the switch created the association in its index table.

The full Microsoft Systems Management Server (SMS) version of Network Monitor (Netmon) binds to the adapter below the WLBS NDIS miniport driver and has a security feature that emits Logical Link Control (LLC) frames when capturing data to warn other computers in the local area network (LAN) that packets are being sniffed. Because these LLC frames originate from the Netmon driver below the WLBS driver, WLBS has no chance to mask the VMAC address. The switch would then add an entry in its Index table mapping the VMAC address to the port from which it received the LLC frame. Packets with a destination of the virtual IP address would only reach the host that sent the LLC frame. This host would not service clients that are supposed to be serviced by the other cluster hosts.



RESOLUTION
Do not use the SMS version of Network Monitor in a WLBS cluster host if cluster operations rely on WLBS masking the cluster VMAC address.



WORKAROUND
To work around this problem, use the built-in version of of Network Monitor that comes with Microsoft Windows NT Server. This version of the Network monitor is the Netmon Standard version.



MORE INFORMATION
This behavior does not occur in Microsoft Windows 2000 because the SMS version of Netmon binds to the Network Load Balancing (NLB) component.

For additional information, click the article number below to view the article in the Microsoft Knowledge Base:

193602 Configuration Options for WLBS Hosts Connected to Layer 2 Switch

Keywords: kbnetwork kbprb KB258609

-

[mailto:TECHNET@MICROSOFT.COM Send feedback to Microsoft]

© Microsoft Corporation. All rights reserved.