RSPAN VLAN. SPAN can create a large amount of traffic. When you're monitoring VLANs composed of multiple gigabit interfaces, the SPAN traffic can easily overwhelm a single gigabit RSPAN link. Whenever possible, set up a dedicated RSPAN VLAN link between the switches. Voice VLAN Voice VLAN allows the 3750 to configure a Cisco IP phone that's connected to the switch. The switch uses CDP to transfer information to the phone configuration re- garding Class of Service (CoS) and the VLANs to be used for voice and data traffic. By default, this feature is disabled, which results in the phone not receiving configuration instructions from the switch. In this case, the phone will send voice and data over the default VLAN (VLAN 0 on the phone). 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 configure 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 con- figure 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 CoS values being sent by the phone. You do this with the mls qos trust cos interface command. The mls qos trust cos interface command will not take effect unless you globally enable quality o