Hi, thanks for your reply !!
just want to clarify the solution here,
-On this case we have a router on the local site and local DC (where the backup controller exists),
(1) If we configure the controller as L2, then we need to configure the local router as the gateway for User VLAN, and we cannot use the same vlan id on the backup controller.
So we need to configure a new vlan and subnet on the backup controller (with the vlan gateway on the local DC router), let say a bigger subnet to cover all of the N sites failover.
When faiolover occured, the AP will reboot and user will do new DHCP discovery for new VLAN
(2) if we configure the controller as L3, then we can use the same vlan with the local site, all we need to consider is the Routing configuration from each side
thanks