You have VoWiFi running on FlexConnect AP with centrally-switched SSID. Which configuration must be performed to preserve marking for central-switching SSID?

With a centrally switched SSID, client traffic is forwarded via CAPWAP tunnel to the WLC and then placed onto the wired network as standard IP packets carrying Layer 3 DSCP markings. Configuring 'Trust DSCP' on the switch port connected to the WLC ensures the switch honors those IP header QoS values rather than remarking or zeroing them out.

Centrally switched SSIDs fully support VoWiFi QoS and do not need to be converted to locally switched; CoS is a Layer 2 mechanism that does not apply to the DSCP values carried in the IP headers of centrally switched traffic.

Trust CoS instructs the switch to honor IEEE 802.1p Layer 2 priority bits, which is appropriate for locally switched traffic at the AP level; centrally switched traffic requires Trust DSCP because the relevant QoS field is in the Layer 3 IP header.

Without explicitly enabling Trust DSCP on the switch port facing the WLC, the switch applies its default remarking policy and overwrites the DSCP values, breaking end-to-end QoS for VoWiFi calls on the centrally switched SSID.