Administrator Guide

Table Of Contents
VLANs 759
When an 802.1p priority is associated with a Voice VLAN, then the priority
information is passed onto the VoIP phone using the LLDP-MED or CDP
protocol, along with the Voice VLAN ID, if any. With this method, the
voice data coming from the VoIP phone is tagged with VLAN 0 (or the
configured Voice 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 the operator-configured priority from the IP phone.
By default, the switch is configured to trust the 802.1p priority for traffic
received from any device, including IP phones. If the port is untrusted, voice
traffic is remarked to CoS priority 5 and is classified into CoS queue 2. If trust
mode is enabled, voice packets are not remarked and are classified per the
switch configuration. Voice traffic is identified by the IP phone’s MAC
address when Voice VLAN is configured. This helps to ensure that voice
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.
CDP/LLDP packets are transmitted and received by the switch regardless of
the 802.1X port state. This allows some VoIP phones to self-configure using
the received Voice VLAN information.
Critical Voice VLAN
Dell EMC Networking switches support critical Voice VLAN. Re-
authenticating voice devices remain in the Voice VLAN when critical Voice
VLAN is enabled and no authentication server is reachable. Voice devices will
be re-authenticated when a RADIUS server is reachable. MAB device re-
authentication is suppressed when critical Voice VLAN is enabled and no
RADIUS server is reachable.