Most fence devices cut the power to the target. By contrast, fence devices that perform fabric fencing cut off a node’s access to some critical resource, such as a shared disk or a network switch.

With fabric fencing, it is expected that the cluster will fence the node, and then a system administrator must manually investigate what went wrong, correct any issues found, then reboot (or restart the cluster services on) the node.

Once the node reboots and rejoins the cluster, some fabric fencing devices require that an explicit command to restore the node’s access to the critical resource. This capability is called unfencing and is typically implemented as the fence agent’s on command.

If any cluster resource has requires set to unfencing, then that resource will not be probed or started on a node until that node has been unfenced.