Controller Based WLANs

Fast Failover controller roles and working:

AP to rapidly fail over from a active to a standby controller without
needing to rebootstrap, and significantly reduces network downtime and client traffic disruption during network
upgrades or unexpected failures.

Active: Controller is Active and is serving APs.
Dual: Controller serves some APs and acts as a standby controller for other APs (1+1)
Standby: Controller does not serve APs, only acts as a Standby in case of failover (1:1 or N:1)

Working:
 
If First and second set of APs are brought up with C3 and C6 as their respective LMS. The Standby controller will be learnt as C2(1 st available non active controller in the HA group)

Third set will terminate on its LMS C1 and will get C2 IP as standby. If C2 is not reachable/Maxed out it will get C3 as standby after the AP reports an error to C1. If C3 is not reachable it will continue down the list.

Note: The Standby can be Learnt only from the LMS and once learnt will be saved on the AP until there is a heartbeat failure.