SoftEther VPN User Forum

The Cluster is a big solution to allow scalability, but not to have availability. What about availability if Cluster Controller is down for failure or for maintenance ?

You can prepare secondary controller sever.
For switching these servers, you can use external redundant service.

Secondary controller sever share configuration with Primary Controller Server ? If not this solution is not applicable in production, as one must have 2 parallel configurations to maintain and at failover time many inconsistency an problems can arise.

You can synchronize a configuration file by some batch.

Do you mean an Active/Passive cluster configuration ? In an Active/Active cluster configuration with failover on the remaining nodes cannot be active the same configuration file on different nodes. Each node has its own configuration and know other node configuration to take place for failed nodes at failover time. This can be a robust cluster solution without have unused resources.