Hi Sam,
One quick note... Deployments that require master controller redundancy should continue to use an existing VRRP redundancy solution.
But it is possible to use HA configure HA in Master-standby mode. It is not recommemded to put the Master or the Backup-Master controller as HA standby.
If the AP reboots, during the master-discovery, it has to first come up on the Master controller. If the Master is an HA standby, it may not accept the primary tunnel from the AP. So the AP will not come up on the Master after the reboot if the Master controller is on Standby mode. I just tested this quickly to verify it.
If you would like to use HA with Master-Redundancy, I would recommend, Local as HA-Active, Master and the Backup-Master as HA-Dual.
Make sure that your HA group-profile is configured in this order.
HA group-member IP address <Local-ip> active
HA group-member IP address <Master-IP> dual
HA group-member IP address <Standby-IP> dual
The LMS IP on the AP system profile as the Local controller's IP.
Thanks,
Rajaguru Vincent