Configuring VLANs 559
Segregating Traffic with the Voice VLAN
You can configure the switch to support Voice VLAN on a port that is
connecting the VoIP phone. Both of the following methods segregate the
voice traffic and the data traffic in order to provide better service to the voice
traffic.
• When a VLAN is associated with the Voice VLAN port, then the VLAN ID
information is passed onto the VoIP phone using either the LLDP-MED or
the CPD mechanism, depending on how the phone is identified: if it is
identified via CDP, then the VLAN assignment is via CDP and if it is
identified via LLDP-MED, then the VLAN assignment is via LLDP-MED.
By this method, the voice data coming from the VoIP phone is tagged with
the exchanged VLAN ID. Untagged data arriving on the switch is given the
default PVID of the port, and the voice traffic is received tagged with the
predefined VLAN. As a result, both kinds of traffic are segregated in order
to provide better service to the voice traffic.
• When a dot1p priority is associated with the Voice VLAN port instead of a
VLAN ID, then the priority information is passed onto the VoIP phone
using the LLDP-MED or CDP mechanism. By this method, the voice data
coming from the VoIP phone is tagged with VLAN 0 and with the
exchanged priority; thus regular data arriving on the switch is given the
default priority of the port (default 0), and the voice traffic is received with
a higher priority.
You can configure the switch to override the data traffic CoS. This feature can
override the 802.1 priority of the data traffic packets arriving at the port
enabled for Voice VLAN. Therefore, any rogue client that is also connected to
the Voice VLAN port does not deteriorate the voice traffic.
Voice VLAN and LLDP-MED
The interactions with LLDP-MED are important for Voice VLAN:
• LLDP-MED notifies the Voice VLAN component of the presence and
absence of a VoIP phone on the network.
• The Voice VLAN component interacts with LLDP-MED for applying
VLAN ID, priority, and tag information to the VoIP phone traffic.
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