If the primary unit fails, the backup units stop receiving HA heartbeat packets from the primary unit. If one of the backup units fails, the primary unit stops receiving HA heartbeat packets from the backup unit. In either case, the cluster is considered down until it is reconfigured.
When the cluster goes down, the cluster units still operating send SNMP traps and write log messages to alert the system administrator that a failure has occurred. You can also see the failure on the HA Status page.
Reconfigure the cluster by removing the failed unit from the cluster configuration. If the primary unit has failed, this means configuring one of the backup units to be the primary unit and adding peer IPs for all of the remaining backup units to the new primary unit configuration.
If a backup unit has failed, reconfigure the cluster by removing the peer IP of the failed backup unit from the primary unit configuration.
Once the cluster is reconfigured, it will continue to operate as before but with fewer cluster units. If the failed unit is restored you can reconfigure the cluster again to add the failed unit back into the cluster. In the same way you can add a new unit to the cluster by changing the cluster configuration to add it.