When you enter the mls qos trust cos command on a port, the switch uses the CoS marking on incoming packets in order to put the packet in the right queue. When the packet is resent, the switch makes the DSCP value correspond to the CoS. https://www.cisco.com/c/en/us/support/docs/wireless-mobility/voice-over-wireless-lan- vowlan/116056-technote-qos-00.html#anc3