Administrator Guide

VLANs 709
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. In either case, 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. As a result,
both kinds of traffic may be segregated by operator configuration in order
to provide better service to the voice traffic. Traffic on the Voice VLAN can
be assigned to a specific CoS queue or otherwise given priority using the
normal policy configuration mechanisms.
• When an 802.1p priority is associated with a Voice VLAN port instead of,
or in conjunction with, a VLAN ID, then the priority information is passed
onto the VoIP phone using the LLDP-MED or CDP protocol. With this
method, the voice data coming from the VoIP phone is tagged with
VLAN 0 (or the configured VLAN) and with the configured priority;
regular data arriving on the switch is given the default priority of the port,
and the voice traffic is received with an operator-configured priority.
By default, the switch is configured to trust the 802.1p priority for traffic
received from any device, including IP phones. The voice traffic is identified
by the IP phone’s MAC address when Voice VLAN is configured. This helps
to ensure that the Voice VLAN traffic is resilient in the presence of other
devices on the network such as a desktop computer.
Voice VLAN is incompatible with Auto-VoIP. Disable Auto-VoIP before
enabling Voice VLAN.
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.
Private VLANs
Private VLANs partition a standard VLAN domain into two or more
subdomains. Each subdomain is defined by a primary VLAN and a secondary
VLAN. The primary VLAN ID is the same for all subdomains that belong to a