Microsoft KB Archive/232315

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The information in this article applies to:


 * Microsoft Windows NT Server, Enterprise Edition version 4.0
 * Windows NT Load Balancing Service 1.0

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SUMMARY
For Windows Load Balancing Service (WLBS) to work properly, every packet directed to the virtual IP (VIP) address must reach all WLBS hosts. The WLBS host decides which packet to accept.

MORE INFORMATION
When the cluster network adapter of each WLBS host is connected to a port on a Layer 2 switch (the same switch) instead of a hub, the switch tries to learn the media access control (MAC) address of the computer connected to each of its ports so it can associate a port to a MAC address. Ethernet switches send frames to a MAC address using the port associated with the MAC address.

If a switch associates the cluster's MAC address to one of its ports, WLBS is unable to correctly load-balance traffic. There are two ways you can configure WLBS to prevent a switch from associating the cluster's MAC address:


 * Unicast

When you use unicast, keep the MaskSourceMAC registry value set to its default value of 1. This forces the cluster to use a "dummy" MAC address when sending packets through the switch. The switch maps the dummy MAC address to a port, but sends traffic to the real cluster MAC address to all ports in the switch. If a switch does not have a MAC address associated to a port, it sends the frames to all ports.
 * Multicast

The cluster uses a multicast MAC address. The switch does not associate multicast MAC addresses to a port, so the switch sends frames to this MAC address on all ports.

When the local router needs to send a packet to the VIP, the local router uses address resolution protocol (ARP) to determine the VIP's MAC address.

WLBS replies to these ARP requests. When you mask the source MAC address, the ARP response from WLBS has a dummy source MAC address in the Ethernet frame, but contains the MAC address of the correct cluster in the ARP header.