Users Guide

VLANs 783
Figure 20-2. Double VLAN Tagging Network Example
Voice VLAN
The Voice VLAN feature enables switch ports to carry voice traffic from IP
phones with an administrator-defined priority. When multiple devices, such
as a PC and an IP phone, are connected to the same port, the port can be
configured to use one VLAN for voice traffic and another VLAN for data
traffic. Multiple IP phones per port are supported.
Voice over IP (VoIP) traffic is inherently time-sensitive: for a network to
provide acceptable service, low latency is vital. Voice VLAN enables the
separation of voice and data traffic coming onto the port and can provide
expedited forwarding of Voice VLAN traffic. Untrusted ports rewrite voice
packets to use 802.1p priority 5. The voice packets are classified into CoS
queue 2. For trusted ports, voice packets are classified into the CoS queue
associated with the received 802.1p priority, or DSCP value as configured by
the administrator.
A primary benefit of using Voice VLAN is to ensure that the sound quality of
an IP phone is safeguarded from interference when the data traffic on the
port is high. The switch uses the source MAC address of the traffic traveling
through the port to identify the IP phone data flow.