You should issue the mls qos trust cos interface configuration command on the switch port connected to an IP phone to configure that port to trust the class of service (CoS) values of incoming voice and data traffic. CoS values are used to prioritize voice and data traffic so the delay sensitive voice traffic receives preferential treatment on the network. By default, the switch port does not trust the CoS values of incoming traffic and reclassifies the traffic with the port's default CoS value of 0. The mls qos trust cos command configures the switch to trust the CoS value of both voice traffic and data traffic that is sent by an IP phone.
The multilayer switching (MLS) port trust feature can be used to examine the CoS or differentiated services code point (DSCP) value to classify incoming traffic. To configure the MLS port trust state, you should issue the mls qos trust command. The syntax for configuring the MLS port trust state is mls qos trust [cos | dscp | ip precedence].
If you do not specify one of the keywords or if Quality of Service (QoS) has been disabled globally, the mls qos trust command defaults to dscp, which classifies incoming traffic according to the DSCP values in the packet header. If you use the ip-precedence keyword, the incoming packets are classified according to the type of service (ToS) bits in the packet header.
Issuing the switchport priority extend cos interface configuration command on the switch port to which the IP phone is connected configures the IP phone to override the priority of the data packets it receives from the host and assigns new CoS values to the host generated packets. Using the switchport priority extend cos command reclassifies incoming data packets with the default CoS value of 0. Thus an IP phone can prevent the computer from exploiting a high-priority data queue.
The switchport priority extend trust interface configuration command configures an IP phone to trust the CoS value of incoming data packets it receives from the attached computer. The switchport priority extend trust command does not configure the switch port to trust the traffic it receives from an IP phone.
The switchport trunk native vlan tag interface configuration command configures native virtual LAN (VLAN) traffic to be tagged. By default, traffic from the native VLAN is sent untagged. Tagging native VLAN traffic is necessary to enable Layer 2 QoS support on the native VLAN.