Connect the new FortiGate to the cluster and your network as shown in the network diagram at the start of this use case. Making these connections disrupts network traffic as you disconnect and re-connect the heartbeat interfaces. If you have already added switches to connect the heartbeat interfaces, you can connect the new FortiGate without disrupting network traffic.
When you add a third FortiGate to a cluster you need to connect the heartbeat interfaces together using switches. You can use separate switches for each heartbeat interface (recommended for redundancy) or you can use the same switch for both heartbeat interfaces as long as you separate the traffic from each heartbeat interface.
When you connect the heartbeat interfaces of the new FortiGate, the cluster re-negotiates. If you have enabled override on the primary FortiGate and set its priority highest, the primary FortiGate synchronizes its configuration to the new FortiGate. The cluster automatically forms with minimal or no additional disruption to network traffic.
The new cluster will have the same IP addresses as the primary FortiGate. You can log into the cluster by logging into the primary FortiGate CLI or GUI.