Server cluster networks

A network (sometimes called an interconnect) performs one of the following roles in a cluster:

A private network carries internal cluster communication. The Cluster service authenticates all internal communication, but administrators who are particularly concerned about security can restrict internal communication to physically secure networks.
A public network provides client systems with access to cluster application services. IP Address resources are created on networks that provide clients with access to cluster services.
A mixed (public-and-private) network carries internal cluster communication and connects client systems to cluster application services.
A network that is not enabled for use by the cluster (that is, neither public nor private) carries traffic unrelated to cluster operation.

Make certain that the private network is at the top of the Network Priority list for internal node-to-node communication in the cluster. For more information, see To change network priority for communication between nodes.

Tolerating network failure

The Cluster service uses all available private and mixed networks for internal communication. Configure multiple networks as private or mixed to protect the cluster from a single network failure.

If there is only one such network available and it fails, the cluster nodes stop communicating with each other. When two nodes are unable to communicate, they are said to be partitioned. After two nodes become partitioned, the Cluster service automatically shuts down on one node to guarantee the consistency of application data and the cluster configuration. This can lead to the unavailability of all cluster resources.

For example, if each node has only one network adapter, and the network cable on one of the nodes fails, each node (because it is unable to communicate with the other) attempts to take control of the quorum resource. There is no guarantee that the node with a functioning network connection will gain control of the quorum resource. If the node with the failed network cable gains control, the entire cluster is unavailable to network clients.

However, if each node has at least two networks and both networks are configured for internal communication (for example both are mixed networks), the Cluster service can tolerate network failures. In this scenario, the Cluster service can detect a public network adapter failure and fail over all resources that depend on that adapter (through its IP address) to a node where this network is available. This is accomplished because the private network is still functioning properly. If, on the other hand, an adapter on the private network fails, the Cluster service can use the public network for internal communication. This is accomplished because the public network is mixed, allowing both internal and client traffic.

Note that there must be a unique subnet for each network in the Cluster.

Node-to-node communication

The Cluster service does not use public-only networks for internal communication, even if a public network is the only available network. For example, suppose a cluster has Network A configured as private and Network B configured as public, and Network A fails. The Cluster service does not use Network B because it is public only; thus, the nodes stop communicating and the cluster breaks apart.

For more information on the concepts in this topic, see: