Dell PowerConnect 8024 User Configuration Manual page 546

The Voice VLAN feature can be enabled on a per-port basis. This feature
supports a configurable voice VLAN DSCP value. This value is later retrieved
by LLDP when the LLDPDU is transmitted, if LLDP has been enabled on
the port and the required TLV is configured for the port.
Identifying Voice Traffic
Some VoIP phones contain full support for IEEE 802.1X. When these phones
are connected to a port that uses 802.1X port-based authentication, these
phones authenticate and receive their VLAN information from LLDP-MED.
However, if a VoIP phone has limited support for 802.1X authentication it
might try to authenticate and fail. A phone with no 802.1X support would not
attempt to authenticate at all. Instead of placing these phones on an
unauthenticated or guest VLAN, the switch can automatically direct the VoIP
traffic to the Voice VLAN without manual configuration.
The switch identifies the device as a VoIP phone by one of the following
protocols:
• Cisco Discovery Protocol (CDP) or Industry Standard Discovery Protocol
(ISDP) for Cisco VoIP phones
• DHCP vendor-specific options for Avaya VoIP phones
• LLDP-MED for most VoIP phones
NOTE:
By default, ISDP is enabled globally and per-interface on the switch.
LLDP-MED is disabled on each interface by default. Port-based authentication
using 802.1X is also disabled on each port by default.
After the VoIP phone receives its VLAN information, all traffic is tagged with
the VLAN ID of the Voice VLAN. The phone is considered to be authorized
to send traffic but not necessarily authenticated.
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.
546
Configuring VLANs