Cisco Switching Basics: Configuring Voice VLANs

Today we are going to talk about the Voice VLAN. The Voice VLAN feature enables the VLAN to carry the voice traffic. So you have a switch with is connected to the IP Phones, it can be of Cisco Avaya or any other and is connected to the specified VLANs or you can name them Voice VLANs in your network. 

So when switch is connected to the IP phones, the connected switch sends the voice traffic with layer 3 IP precedence and Layer 2 class of service in short name as COS values which are set as default of value 5. 

That is the reason 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 CoS. 

We can configure an access port with an attached Cisco IP Phone to use one VLAN for voice traffic and another VLAN for data traffic from a device attached to the phone. We can configure access ports on the switch to send Cisco Discovery Protocol (CDP) packets that instruct an attached phone to send voice traffic to the switch in any of these ways

• In the voice VLAN tagged with a Layer 2 CoS priority value
• In the access VLAN tagged with a Layer 2 CoS priority value
• In the access VLAN, untagged (no Layer 2 CoS priority value

Fig 1.1- Sample Topology Switch & IP Phone

There are some guidelines if you are going to configure the Voice VLAN in your network. 

• The voice VLAN should be present and active on the switch for the IP phone to correctly communicate on the voice VLAN. 
• Before you enable voice VLAN, we recommend that you enable QoS on the switch by entering the mls qos global configuration command and configure the port trust state to trust by entering the mls qos trust cos interface configuration command.
• we must enable CDP on the switch port connected to the Cisco IP Phone to send the configuration to the phone. 
• The Port Fast feature is automatically enabled when voice VLAN is configured. When you disable voice VLAN, the Port Fast feature is not automatically disabled. 
• The Cisco IP Phone and a device attached to the phone cannot communicate if they are in the same VLAN and subnet but use different frame types because traffic in the same subnet is not routed 

Configuring the Voice VLANs. Below are the configuration to configure the port and then setting the priority to the interfaces which is configured as a part of voice VLANs

Configuring Port
NDNA# configure terminal
NDNA(config)# interface Fa0/8
NDNA(config-if)# mls qos trust cos
NDNA(config-if)# switchport voice vlan dot1p
NDNA(config-if)# end 

Setting Priority
NDNA# configure terminal
NDNA(config)# interface Fa0/8
NDNA(config-if)# switchport priority extend trust
NDNA(config-if)# end