Cluster Communications

It is strongly recommended that a cluster have more than one network connection. A single network connection threatens the cluster with a single point of failure. There are three options for network configurations, private, public, and mixed. Each network configuration requires its own dedicated network card.

Private Network Cluster nodes need to be consistently in communication over a network to ensure that both nodes are online. Cluster service can utilize a private network that is separate from client communications. Once a connection is configured as a private network it can only be used for internal cluster communications, and is known as a private network or interconnect. The private network will be the default route for node-to-node communication. The cluster cannot use a private network for client-to-node communication. Heartbeats Each node in a cluster periodically exchanges sequenced User Datagram Protocol (UDP) datagrams with the other node in the cluster to determine if it is up and running correctly, and to monitor the health of a network link. This process is referred to as sending heartbeats.

Public Network The public network connection is used as a dedicated client-to-node communication network. The cluster cannot use the public network for node-to-node communication.

Mixed Network Another configuration option is to create a network that is used for both private and public communication. This is called a mixed network. Using a mixed network does not change the recommendation for two networks. Important: The recommended configuration for server clusters is a dedicated private network for node-to-node communication and a mixed network. The mixed network acts as a backup connection for node-to-node communication should the private network fail. This configuration avoids having any single point of network failure.

Cluster Network Requirements

A cluster network has two primary types of communication A cluster network has two primary types of communication. The private communication provides online status and other cluster information to the nodes. The public communication provides information between the client and virtual servers. It is recommended that private network communications be physically separated from public network communications, but have the ability to use the mixed network to eliminate a single point of failure. An alternative network type a mixed network, uses the public network for both private and public communications. The advantage of a mixed network is that private network communications can be failed over to the public network. If you dedicate a network to client-to-node communications, node-to-node communications cannot fail over to that network. The mixed network configuration is the preferred configuration for a public network. Network adapters, known to the cluster as network interfaces, attach nodes to networks. You configure what types of communication will travel across the networks by using the cluster administration tools.

Cluster Network Requirements: Private Network The private network, also known as the interconnect in a server cluster, will potentially carry the following five types of information: Server heartbeats. Each node in a cluster exchanges IP packets (approximately every 1.2 seconds) with the other node in the cluster determining if both nodes are running correctly. The first node of the cluster to come online is the node that is responsible for sending out the heartbeats. The second node begins to send heartbeats to inform the first node that the second node has come online. If a node does not respond to the heartbeat, the working node identifies the unresponsive node as having failed. If a node fails to detect six successive heartbeats from another node, the other node assumes that the unresponsive node is offline and tries to take ownership of the resources that the nonresponding node owns. Note that failure to detect a heartbeat message can be caused by node failure, network interface failure, or network failure. Replicated state information. Every node in the cluster uses replicated state information to communicate which cluster groups and resources are running on all of the other nodes. Cluster commands. Cluster commands manage and change the cluster configuration. An example of a cluster command would be any node-to-node communications regarding adding or removing a resource or failing over a group. Application commands. Cluster-aware applications send application commands through the interconnect to communicate with copies of an application running on multiple servers. Application data. Application data is when cluster-aware applications share data between nodes. Note: If the private network fails over to the public network, the Cluster service employs packet signing for node-to-node communications to provide additional security over the network.

Cluster Network Requirements: Public Network The public network connection extends beyond the cluster nodes and is used for client-to-cluster communication. The public network cannot function as a backup connection for node-to-node communication should the private network fail. The network interface cards (NICs) for the public network must be on the same subnet.

Cluster Network Requirements: Mixed Network The typical server cluster will have one NIC on each server that is designated for internal communications (cluster only), and one or more other NICs designated for all communications (the mixed network serving both cluster and client). In such a case, the cluster-only NIC is the primary interconnect, and the mixed network NIC(s) serves as a backup if the primary ever fails. Cluster service can only do this for the primary cluster interconnect. That is, it provides the ability to use an alternate network for the cluster interconnect if the primary network fails. This eliminates an interconnect NIC from being a single point of failure. There are vendors who offer fault tolerant NICs for Windows 2000 Advanced Server, and you can use these for the NICs that connect the servers to the client network.