In Microsoft Cluster Server (MSCS) in Windows NT Server, Enterprise Edition, a message that the Cluster service running on one node regularly sends to the Cluster service on the other node to detect a failure.
Nodes in a cluster communicate status information with each other using the Cluster service, a Microsoft Windows NT service on Cluster Server. These messages appear as network traffic between the two nodes in the cluster on the dedicated network connection between the nodes, which is called the private network of the cluster.
If the heartbeat is lost over the private connection, Cluster Server reverts to a public network connection for its heartbeat and other Cluster service traffic. You can configure the polling interval for the heartbeat using Cluster Administrator.