Adjusting multicast parameters

Applies To: Windows Server 2003, Windows Server 2003 R2, Windows Server 2003 with SP1, Windows Server 2003 with SP2

Adjusting multicast parameters

Network Load Balancing uses one of two methods to enable the cluster adapters on all cluster hosts to simultaneously receive network traffic for the cluster's IP addresses:

  • When multicast support is enabled in the Network Load Balancing Properties dialog box, Network Load Balancing adds a multicast media access control (MAC) address to the cluster adapters on all cluster hosts. At the same time, the cluster adapters retain their original MAC addresses.

  • When multicast support is disabled in the Network Load Balancing Properties dialog box (causing the host to revert to unicast mode), Network Load Balancing instructs the driver belonging to the cluster adapter to change the adapter's MAC address to the cluster MAC address. This is the same address used on all cluster hosts. (Note that some network adapters do not support changing their MAC addresses.) When this occurs, the cluster adapters are no longer addressable by their original MAC addresses.

Advantages of using multicast support

Multicast support has several advantages, such as removing the inherent limitations of a single network adapter, as explained in Single network adapter limitations. Network Load Balancing handles the resolution of the cluster's IP address to its associated multicast cluster MAC address used in ARP on your router. Note that in rare cases the use of a multicast MAC address might not be supported by the ARP implementation on a given router. If this problem arises, the cluster will not be accessible from outside the local subnet. To correct this problem, you must create a static ARP entry within the router. To determine how to create a static ARP entry, see the documentation for your router. For more information, see Using a router in Network Load Balancing system requirements.

Avoiding switch port flooding

Some network adapter drivers experience increased CPU load percentages (by 5 percent or more depending on network traffic) when handling multicast MAC addresses instead of unicast addresses. If your cluster hosts have unusually high CPU load percentages when using Network Load Balancing multicast support, disable this feature to see if it is the source of the problem. If switch flooding occurs, that is, your non-cluster hosts receive traffic destined only for the cluster hosts, then you can enable Internet Group Management Protocol (IGMP) support. For more information on avoiding switch port flooding, see Limit switch port flooding. For more information on enabling IGMP support, see Enable Internet Group Management Protocol (IGMP) support.


  • Multicast support is not enabled by default. To enable it, see Enable multicast support.

  • When Network Load Balancing is enabled for use with a network adapter in multicast mode, a manually configured MAC address that may have been optionally assigned to the adapter will be removed after Network Load Balancing is later disabled. When the adapter is subsequently reinitialized (typically when the computer is restarted), the network adapter will revert to its factory-assigned MAC address. To avoid this problem, be sure to reassign the adapter's manually configured MAC address after disabling Network Load Balancing. For more information, see Remove a host from a Network Load Balancing cluster.