Using Voice VLAN Command with Cisco IP Phones

What switchport voice vlan command does is that it will be used with an access vlan command in a Cisco access switch so that the VLAN specified in voice vlan command will be tagged and the VLAN used in access vlan command will be untagged and forwarded to the PC which is attached to the IP phone. Truely this will make this port a trunk at the back end.

Cisco IP phones will identify this tagged VLAN via CDP and will move ahead with registering with a CUCM. Let's do a practical where the commands can be misconfigured and let's see how will devices respond.

In the diagram it shows e0/0 is connected to phone but truly Fa0/7 is the port connected to the phone.
Also note that VLAN 810 is a data VLAN and 904 is a voice VLAN..

Let's see the 1st combination which is the default best practice.

Will work as expected.
Phone will register in VLAN 904
PC will be able to use VLAN 810







2nd combination where both voice VLAN and data VLAN are configured to same data VLAN.

Both the phone and PC will use same VLAN and if VLAN 810 has routing to the CUCM, no problem will occur.







3rd combination where only the access vlan command is configured.

Again both the phone and PC will use same VLAN and if VLAN 810 has routing to the CUCM, no problem will occur.






4th combination where the port is configured as a trunk, and a native VLAN is configured.

This time again looks like the above configurations where the voice VLAN is tagged and the data VLAN is untagged but what happens is again both the phone and PC will use same VLAN and if VLAN 810 has routing to the CUCM, no problem will occur.