In the MQTT Set configuration, there is an option to define both Primary and Secondary servers within a single set. My understanding is that if the Primary server fails, Ignition should automatically switch over to the Secondary server. In this case, is an external load balancer required or not?
Seeing as it's a failover, I would say a load balancer is not needed. A load balancer is to direct incoming traffic to multiple servers to distribute processing load.
The failover is all or nothing since only one server is doing the work at any given time, no balancing needed.