er 18: Catalyst 3750 Features Cisco IP phones such as the model 7960 have built-in three-port switches. Port 1 on the built-in switch is the connection to the upstream switch (the 3750 we'll config- ure here). Port 2 is the internal connection to the phone itself. Port 3 is the external port, which usually connects to the user's PC. By using the switchport voice vlan interface command, you can have the switch configure an IP phone that is connected to the interface being configured. You can specify a VLAN for voice calls originating from the phone, or you can have the switch tell the phone to use the regular data VLAN for voice calls (with or without setting CoS values): 3750(config-if)# switchport voice vlan ? <1-4094> Vlan for voice traffic dot1p Priority tagged on PVID Don't tell telephone about voice vlan untagged Untagged on PVID To set the VLAN, specify a VLAN number. The dot1p option tells the phone to set CoS bits in voice packets while using the data VLAN. The untagged option tells the phone to use the data VLAN without setting any CoS values. To take advantage of Voice VLANs, you need to tell the switch to trust the values being sent by the phone. This is done with the mls qos trust cos interface command. The mls qos trust cos interface command will not take effect unless you globally enable 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 (usu- ally, the user's PC). This feature is configured with the switchport priority extend interface command. The options are cos and trust. When using the cos option, you may set the CoS field to whatever CoS value you like: 