How Cluster Integrity Is Monitored in WLBS

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Article ID: 232711 - View products that this article applies to.
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Cluster integrity is monitored by the periodic exchange of proprietary messages between cluster members; these messages are called heartbeats. Each cluster host periodically sends a heartbeat from its virtual network adapter to the segment in which the cluster's virtual network adapters reside to let the other hosts know that it is "alive."

When the Windows NT Load Balancing Service (WLBS) cluster is configured to use a unicast media access control (MAC) address, the heartbeat is sent to the Ethernet broadcast address, FFFFFFFFFFFF. When the cluster is configured to use a multicast MAC address, the heartbeat is sent to the cluster's multicast MAC address. The virtual network adapter of each cluster host must be in the same network segment or subnet as the other cluster hosts' virtual network adapters to be able to receive the heartbeat from each other.

By default, the time period a cluster host waits before sending the next heartbeat is 1 second. This can be customized by adjusting the AliveMsgPeriod registry parameter.

By default, a cluster host is declared inactive by the other hosts if its heartbeat is missed for five periods. You can customize this value by adjusting the AliveMsgTolerance registry parameter.

The intracluster traffic that is created by the heartbeat is miniscule. It is a function of the cluster size, not of the number of users or offered load. The size of the heartbeat message is 1.5 KB. It is stamped with a version number and newer versions are insured to be backward compatible.


The heartbeat's Ethernet frame type is 0x886f (IEEE registered). The frame type used in previous versions of WLBS is 0xbf01. In a mixed cluster composed of WLBS and previous versions of WLBS, WLBS reverts to frame type 0xbf01.


For additional information, please see the following article in the Microsoft Knowledge Base:
193601 Registry Parameters for Windows NT Load Balancing Service


Article ID: 232711 - Last Review: November 1, 2006 - Revision: 2.2
  • Microsoft Windows NT Server 4.0 Enterprise Edition
  • Microsoft Windows NT Load Balancing Service
kbinfo kbnetwork KB232711

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