Users Guide

796 VLANs
and the voice devices have access to the voice VLAN regardless of
authentication state. The phone must tag the packets with the voice VLAN
sent via LLDP-MED/CDP when the port is configured in access mode.
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 option 176 for Avaya VoIP phones
• LLDP-MED for many VoIP phones
• For ports configured for 802.1X or MAB authentication, an Access-Accept
received from the AAA service with a vendor-proprietary VSA device-
traffic-class = voice identifies the device as a VoIP device.
• DHCP/ISDP/CDP/LLDP information is not used to identify VoIP devices
for assignment to the voice VLAN.
QoS and Voice VLAN
The switch can be configured to support Voice VLAN on a port that is
connected to a VoIP phone. Both of the following methods separate the voice
traffic and the data traffic in order to provide better service to the voice
traffic.
• When a VLAN is associated with a Voice VLAN-enabled port without a
configured 802.1p priority, then the VLAN ID information is passed onto
the VoIP phone using the LLDP-MED or CDP protocols. It is
recommended that both CDP and LLDP-MED be enabled for maximum
device interoperability. In either case, the voice data coming from the VoIP
phone is tagged with the exchanged VLAN ID (if configured). Untagged
data arriving on the switch is processed on the default or dynamically
assigned PVID of the port. As a result, both kinds of traffic may be
separated 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.
1
NOTE: By default, ISDP is enabled globally and per-interface on the switch.
LLDP-MED is enabled on each interface by default. Port-based authentication
using 802.1X is disabled on each port by default.