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Network communication between nodes

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

Network communication between nodes

Communication between the nodes of a cluster enables the Cluster service to detect node failures and status changes, and to manage the cluster as a single entity. You can set up multiple networks for this node-to-node communication so as to eliminate a single point of failure.

Heartbeats

The Cluster service, running on each node of the cluster, keeps track of the current state of the nodes within a cluster and determines when a group and its resources fail over to an alternate node. This communication takes the form of messages that are sent regularly between each node's Cluster service. These messages are called heartbeats.

In addition to heartbeat traffic, some network traffic occurs whenever changes are made to the cluster. For example:

  1. During startup, node-to-node communication occurs for the purpose of synchronizing the cluster databases that reside on each node. The cluster database stores configuration information about the cluster resources.

  2. Node-to-node communication to verify node failures during cluster configuration changes.

As the number of nodes in a server cluster increases, the node-to-node communication rises significantly. In Windows Server 2003, Enterprise Edition or Windows Server 2003, Datacenter Edition, for server clusters with 3 or more nodes, multiple unicast messages for two classes of intracluster traffic are replaced by single multicast messages. This reduces the intracluster traffic, resulting in lower network bandwidth consumption and improved node performance. The configuration of the server cluster must meet the following conditions for heartbeat multicasting:

  • The number of nodes that are members of the server cluster (rather than the number of nodes that are currently up and actively participating in the cluster) must be 3 or greater.

  • All the nodes in the server cluster must be running Windows Server 2003, Enterprise Edition or Windows Server 2003, Datacenter Edition.

Important

  • Both conditions above refer to nodes configured in the cluster membership rather than nodes that are currently up and actively participating in the cluster.

  • 2-node server clusters use unicast, not multicast, messaging for all intracluster traffic.

  • If you operate a server cluster of 3 or more nodes as a mixed version cluster (that is, Windows Server 2003, Enterprise Edition or Windows Server 2003, Datacenter Edition is installed on some nodes, and Windows 2000 on others), then the cluster as a whole will send unicast, not multicast, intracluster messages.

The two kinds of intracluster traffic affected by heartbeat multicasting are:

  • Heartbeat messages sent between nodes.

  • Node-to-node communication to verify node failures during cluster configuration changes.