• Two identical appliances (the same hardware model and same firmware version).

• Redundant network topology: if an active node fails, physical network cabling and routes must be able to redirect web traffic to the other member.

• At least one physical port on both HA appliances connected directly, via a crossover cables or through a Layer 2 switch.

• Heartbeat and synchronization traffic between cluster nodes occur over the physical network ports you specify. If switches are used to connect heartbeat interfaces between nodes, the heartbeat interfaces must be reachable by Layer 2 multicast. HA traffic uses multicast UDP on port numbers 6065 (heartbeat) and 6056 (synchronization). The HA multicast IP address is 239.0.0.1; it is hard-coded, and cannot be configured.

• Each appliance must be licensed.