Core Issue
The voice VLAN feature is disabled by default on a Catalyst switch.
Resolution
When the voice VLAN feature is enabled, it allows voice traffic from the attached IP phone and data traffic from a daisy-chained PC to be transmitted on different VLANs. This capability provides flexibility and simplicity in IP address allocation and prioritizing voice over data.
Untagged traffic is sent according to the default Class of Service (CoS) priority of the Catalyst switch port. The CoS value is trusted for 802.1p or 802.1q tagged traffic.
The voice VLAN feature enables access ports to carry IP voice traffic from an IP phone. The switch can connect to a Cisco 7960 IP phone and carry IP voice traffic. Because the sound quality of an IP phone call can deteriorate if the data is unevenly transmitted, the switch supports QoS based on IEEE 802.1p CoS.
QoS uses classification and scheduling to send network traffic from the switch in a predictable manner. The Cisco 7960 IP phone is a configurable device and you can configure it to forward traffic with an 802.1p priority. You can configure the switch to trust or override the traffic priority assigned by a Cisco 7960 IP phone.
The Cisco 7960 IP phone contains an integrated three-port 10/100 switch. The ports are dedicated connections to these devices:
- Port 1 connects to the switch or other VoIP device.
- Port 2 is an internal 10/100 interface that carries the IP phone traffic.
- Port 3 connects to a PC or other device.
For more information on configuring voice VLANs, refer to these documents:
