Have a look at the attached VRD, this is detailed in greater depth allong with packet anaysis.
There are two major types of communication between APs in a cluster:
- L2 Broadcast messages for cluster maintenance and roaming
Master AP sends out a Layer 2 “beacon” message every second to notify that the master AP is currently active. This helps in new APs discovering the master and join the cluster & existing APs detecting master failover and take over as master of cluster.
- L3 unicast messages between IAPs (master and slave)
UDP messages on port 8211 between master and slave APs for config sync, firmware upgrade and control-plane messaging between APs.
Link to the VRD https://community.arubanetworks.com/aruba/attachments/aruba/Aruba-VRDs/76/2/Aruba_Instant_VRD_2016.pdf