Cisco 7925G Series Deployment Manual page 54

If using layer 3 mobility, then symmetric 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 Wireless IP Phone 7925G Series Deployment Guide 54