Thursday, 31 December 2009


First To Understand Voice VLAN

The voice VLAN feature enables you to configure a switch port ports to carry voice traffic from IP phone. When the switch is connected to a Cisco IP Phone, the phone sends voice traffic with Layer 3 IP precedence that give voice a priority on Data. Because the sound quality can be compromised if Data is being sent unevenly. and you need to configure a switch to trust the priority assigned by Cisco IP phone

The Cisco IP Phone contains an integrated 10/100 switch as shown below

So Connectivity would be something like this

Port 1 connects to the PoE Switch
Port 2 (access port) connects to a PC or other device.

Configuration and Tuning

After connectvity You need to configure an access port attached with Cisco IP Phone to carry voice through One VLAN and data traffic from another VLAN So here comes in play Cisco Discovery Protocol (CDP) packets that instruct an attached phone to send voice traffic to the switch in appropriate LAN  with IP Precedence priority Vlaue 

Default Layer 3 IP Precedence Value for Voice is 5 and 3 for Control  traffic

Note that Voice VLAN is only supported on access ports and not on trunk ports, though this configuration is allowed and no error is shown.

you must be careful that Voice Vlan exist on the Switch (by issuing th following Command in Exec Mode SHOW VLAN )before setting up the port for IP Phone.  I hope no one pops up and ask how to create Voice Vlan ... :)

You need to enable QoS on the switch ( mls qos ) and configure the port trust by entering (mls qos trust cos) so Switch interface will trust the CoS attached in Header 

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 

So the final config will look like as Below :- 

Switch_A # config t

Switch_A(config)# interface gi 0/1

Switch_A(config-if)# mls qos trust cos         (Enable QOS and Trust the Value of CoS)

Switch_A(config-if)# switchport voice vlan dot1q

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