Understanding Voice VLAN

The voice VLAN feature enables aclearcase/" target="_blank" >ccess 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 sent, the switch supports quality of service (QoS) based on IEEE 802.1P class of service (CoS). QoS uses classification and scheduling to send network traffic from the switch in a predictable manner. For more information on QoS, see Chapter 29, "Configuring QoS." 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 an IP Phone.

The Cisco 7960 IP Phone contains an integrated three-port 10/100 switch as shown in Figure 14-1. The ports provide dedicated connections to these devices:

•Port 1 connects to the switch or other voice-over-IP (VoIP) device.

•Port 2 is an internal 10/100 interface that carries the IP phone traffic.

•Port 3 (access port) connects to a PC or other device.

Figure 14-1 shows one way to connect a Cisco 7960 IP Phone.

Figure 14-1 Cisco 7960 IP Phone Connected to a Switch

When the IP Phone connects to the switch, the access port (PC-to-telephone jack) of the IP phone can connect to a PC. Packets to and from the PC and to or from the IP phone share the same physical link to the switch and the same switch port. For deployment examples that use voice VLANs, refer to the "Network Configuration Examples" section on page 1-9.

Configuring Voice VLAN

This section describes how to configure voice VLAN on access ports. It contains this configuration information:

• Default Voice VLAN Configuration

• Voice VLAN Configuration Guidelines

• Configuring a Port to Connect to a Cisco 7960 IP Phone

Default Voice VLAN Configuration

The voice VLAN feature is disabled by default.

When the voice VLAN feature is enabled, all untagged traffic is sent according to the default CoS priority of the port.

The default CoS value is 0 for incoming traffic.

The CoS value is not trusted for 802.1P or 802.1Q tagged traffic.

The IP Phone overrides the priority of all incoming traffic (tagged and untagged) and sets the CoS value to 0.