Administrator Guide

VLANs 707
Figure 21-2. 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. The priority level enables the
separation of voice and data traffic coming onto the port and can provide
expedited forwarding of voice VLAN traffic.
A primary benefit of using Voice VLAN is to ensure that the sound quality of
an IP phone is safeguarded from deteriorating 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.
The Voice VLAN feature can be enabled on a per-port basis. This feature
supports a configurable voice VLAN DSCP or IEEE 802.1p value. This value
is later transmitted by LLDP when the LLDPDU is transmitted, if LLDP has
been enabled on the port, the DSCP/802.1p value is configured, and the
required TLV is configured for the port.