Explanation/Reference:
* Failover (step 1)
ROS (role orchestration service) triggers the failover operation in two cases:
/Controller failure:
/Device disconnect:
The ROS instance in a controller is notified of a communication failure with network device(s) through the Controller Service notifications. It instantly communicates with all ROS instances in the team to determine if the network device(s) in question are still connected to any of the backup (slave) controllers within the team. If that is the case, it elects one of the slaves to assume the master role over the affected network device(s).
* Failback (step 2)
When the configured master recovers from a failure and rejoins the team, or when the connection from the disconnected device(s) with the original master is resumed, ROS initiates a failback operation in which the master role is restored to the configured master as defined in the region definition.
Reference: HP VAN SDN Controller Administrator Guide