1. From the GUI of the Extreme Cloud Appliance (XCA) go to Administration > System > Interfaces > and either create a new interface of type "Physical" if one doesn't exist, or modify an existing interface of type "Physical" for which you want access points to be able to register. On the screen that allows configuration/modification of an interface on the XCA, check off the box named "Enable Device Registration" (highlighted in red below).
Save that change to the interface.
2. If the APs you want to register with the XCA are in the same VLAN/subnet as the interface for which you enabled device registration above, and if the port(s) that your AP(s) connect to do not have IGMPSnooping enabled (which would supress the SLP multicast protocol APs use to discover controllers in their own local subnet) and as long as there are no firewalls, content filters between the AP(s) and the controller(s), and/or as long as their is no Policy applied to the AP port that would block the ports required for AP <> controller communication, then your AP(s) should register with the controller.
A list of required ports that need to be open between your AP(s) and controller(s) can be found here:https://gtacknowledge.extremenetworks.com/articles/Q_A/What-are-the-tcp-udp-ports-used-between-IdentiFi-Wireless-Controller-and-AP-s
3. If the AP(s) you want to register to your XCA are NOT in the same VLAN/subnet as the Extreme Cloud Appliance, then you must:
-- have a default route defined on your XCA that lists the router that is the next hop away from the XCA interface for which you have defined device registration in Step 1 above: https://gtacknowledge.extremenetworks.com/articles/How_To/How-to-configure-the-default-route-gateway-for-an-Extreme-Cloud-Appliance-XCA-controller-through-the-XCA-GUI
-- have an AP discovery method in place using either DNS or DHCP Option 78 that will "point" the AP(s) to the interface on XCA that has device registration enabled:https://gtacknowledge.extremenetworks.com/articles/How_To/How-To-Configure-an-AP-to-find-the-IdentiFi-Wireless-Controller-from-a-DNS-server-entryhttps://gtacknowledge.extremenetworks.com/articles/How_To/How-To-Configure-an-AP-to-Find-the-IdentiFi-Wireless-Controllerhttps://gtacknowledge.extremenetworks.com/articles/How_To/How-To-Configure-an-AP-to-Find-the-IdentiFi-Wireless-Controller-with-DHCP-Option-78-on-a-Linux-Serverhttps://gtacknowledge.extremenetworks.com/articles/How_To/AP-Distributed-Mode-WiNG-XCA-discovery-Microsoft-DHCP-configurationhttps://gtacknowledge.extremenetworks.com/articles/Solution/Access-points-failing-the-IdentiFi-Controller-discovery-processhttps://gtacknowledge.extremenetworks.com/articles/Q_A/Can-the-IdentiFi-wireless-appliances-DHPC-server-support-advance-DHCP-options
4. If your APs are connecting over a WAN/VPN/Low-Bandwidth link to attempt registration with XCA, you may have to adjust the MTU the APs are configured to use:https://gtacknowledge.extremenetworks.com/articles/How_To/How-to-Configure-APs-to-register-to-ExtremeWireless-or-XCA-controllers-over-WAN-and-or-VPN-from-remote-locations