When a client roams between two access points (APs) that are registered to different controllers with interfaces in the same client subnet, the controllers will exchange mobility control messages over UDP port
16666. This is in accordance with the Designing Cisco Enterprise Wireless Networks (ENWLSD) guidelines.
The purpose of these messages is to ensure seamless roaming, where the client's session and authentication state are transferred from one controller to another without requiring re-authentication or re-association by the client. The mobility control messages facilitate the coordinated transfer of the client database entry from the original controller to the new one, maintaining a consistent experience for the user.