Cisco 9971 Deployment Manual page 63

If utilizing layer 3 mobility, then Symmetric Mobility Tunneling should be Enabled.
In the recent versions, Symmetric Mobility Tunneling is enabled by default and non-configurable.
When multiple Cisco Unified Wireless LAN Controllers are to be in the same mobility group, then the IP address and MAC
address of each Cisco Unified Wireless LAN Controller should be added to the Static Mobility Group Members configuration.
Cisco Unified IP Phone 9971 Wireless LAN Deployment Guide
63