Cisco Support Community
Showing results for 
Search instead for 
Did you mean: 

Welcome to Cisco Support Community. We would love to have your feedback.

For an introduction to the new site, click here. If you'd prefer to explore, try our test area to get started. And see here for current known issues.

New Member

clarification on voice vlan settings.

I need some clarification on setting up voice vlans. The IP phone gives it own data packets a COS value of 5 if it has a PC attached it changes the COS value of packets received from the PC to a value 0?

On the switch settings, what is the difference between 1. “MLS QOS Trust COS” and 2. “MLS QOS Trust device Cisco-phone”

If I use 1, If either Phone and PC or just PC is attached to the port, the switch excepts the COS value on all packets received on this port.

Using 2 if there is no phone on the port the switch does not trust the COS value. Is that about right?

If I use “Switchport voice vlan dot1p” the voice and data traffic is sharing the same vlan?

If I use “Switchport voice vlan dot1q” the voice traffic is on it own vlan?

Any verification of whether I am close or way off the mark would be appreciated. Thanks


Re: clarification on voice vlan settings.

Hi David,

Please have a look at teh following configuration guide for clarity on voice vlan concepts -

-> Sushil

New Member

Re: clarification on voice vlan settings.

Sushil, Thanks for the link, it has cleared up a few things.

Re: clarification on voice vlan settings.

Hi David,

You should configure a switch to instruct an IP phone about what to do with the CoS value coming from the PC.

The "switchport priority extend cos x" will instruct the IP phone to override the CoS value coming from the PC to value x.

The "mls qos trust cos" command will cause the switch to trust the cos value of any incoming frames.

The "mls qos trust device cisco-phone" and "mls qos trust cos" commands together will instruct the switch to trust the cos values of incoming frames only if a Cisco IP phone is connected to the port. The switch verifies this using CDP version 2.

If there in no IP phone connected, the CoS value is not trusted and is overridden to 0.

If you use "switchport voice vlan dot1p" then the voice frames will be put in vlan0. Vlan0 will be tagged with a null tag and will be able to carry the CoS values.

switchport voice vlan dot1q command does not exist.

You can use the "switchport voice vlan x" command, which is the most frequent and reliable application.

In this case the voice frames are put into vlan x (tagged), data frames are put on the native vlan of the access port (untagged).



New Member

Re: clarification on voice vlan settings.

Hello Istvan,

Thanks for the replies. So if I set all my ports as below this will ensure that all data packets have a cos value of 0 with or without a phone attached.

Switchport priority extend cos 0

Mls qos trust device cisco-phone

Mls qos trust cos

Switchport mode access

Switchport voice vlan 100

Switchport access vlan 150

Also I need to route voice vlan 100 onto vlan 150 to access a gateway out. To make sure that voice data retain the cos value should I replace COS with DSCP?

When I used "switchport voice vlan dot1p" I did get errors about not finding vlan 0, what's vlan 0, you can't create it.



Re: clarification on voice vlan settings.

Hi David,

No, this configuration will set the CoS value of the attached PC to 0 and trust the CoS value coming from the IP Phone.

If no IP phone is attached, then it will set all CoS values to 0.

Multilayer swiches use a CoS-to-DSCP mapping table to set the DSCP value from the incoming CoS value.

The default CoS-to-DSCP mapping table is the following (CoS-DSCP)

0-0, 1-8, 2-16, 3-24, 4-32, 5-40, 6-48, 7-56

You can modify this to use DSCP value 46 instead of 40 for CoS 5:

mls qos map cos-dscp 0 8 16 24 32 46 48 56

This will then set the internal DSCP value of the IP packet to 46 when CoS is 5.

As to the switchport voice vlan dot1p: your switch probably doesn't support this feature.

As I said, the usual and reliable method is to use "switchport voice vlan x".