f service (QoS) with the mls qos command. Here is a sample interface configured to use VLAN 100 for data and VLAN 10 for voice. The switch will instruct the IP phone to use VLAN 10 for voice, and will trust the CoS values as set by the phone: interface GigabitEthernet1/0/20 switchport access vlan 100 switchport voice vlan 10 mls qos trust cos Another nice aspect of the voice VLAN feature is that you can have the IP phone alter or trust any CoS values set by the device plugged into its external switch port (usually the user's PC). You configure this feature with the switchport priority extend interface Chapter 19: Catalyst 3750 Features command. The options are cos and trust. When using the cos option, you may set the CoS field to whatever CoS value you like: 3750(config-if)#switchport priority extend ? Override 802.1p priority of devices on appliance trust Trust 802.1p priorities of devices on appliance I prefer to trust the PC's CoS values, as different software on the PC may have different values. For example, the user may wish to run a soft- phone application on the PC. Overriding the CoS values set by this software might lead to voice quality issues for the softphone. Here, I've configured an interface to use VLAN 10 for voice while trusting the CoS values set by the user's PC and phone: interface GigabitEthernet1 1/0/20 switchport access vlan 100 switchport voice vlan 10 switchport priority extend trust mls qos trust COS To see which VLAN is configured as the voice VLAN, use the show interface inter face-name switchport command: 3750#sho int g1/0/20 switchport Name: Gi1/0/20 Switchport: Enabled Administrative Mode: static access Operational Mode: down Administrative Trunking Encapsulation: negotiate Negotiation of Trunking: off Access Mode VLAN: 1 (default) Trunking Native Mode VLAN: 1 (default) Administrative Native VLAN tagging: enabled Voice VLAN: 10 (Inactive) Administrative private-vlan host-association: none Administrative private-vlan mapping: none Administrative private-vlan trunk native VLAN: none A