Cisco 8821 Manual page 61

Enable Link Aggregation (LAG) if utilizing multiple ports on the Cisco Wireless LAN Controller.
Configure the desired AP multicast mode.
If utilizing multicast, then Enable Global Multicast Mode and Enable IGMP Snooping should be enabled.
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.
Cisco Wireless IP Phone 8821 and 8821-EX Wireless LAN Deployment Guide
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