Auto QoS Voip Trust question

[Craddockc]

Hello,

I am a little confused on the auto qos voip trust command. I am reading that it should be used on interior switchports like trunk ports connected to another switch. I guess my question is this: If the auto qos voip command tells the switchport to trust the incoming CoS values, then isnt it redundant to tell the port to "trust" the incoming CoS value when the auto qos voip command is already accomplishing this? Or does this have something to do with trusting DSCP values instead of CoS values? If so, why are we trusting DSCP values at the uplinks and not CoS values coming from the IP Phone? Im so confused. Thanks for any light you can shed on this topic.

Chris.

[Jon Marshall]

Chris

My understanding of this command "auto qos voip trust" was that you enabled it on uplinks to switches or routers. If the port on the switch is a L2 trunk port then you trust CoS, if it is a L3 port then you trust DSCP.

I'm not sure what you mean about the "auto qos voip" command on it's own already doing this.  Can you point to the documentation where you reading this ?

Jon

[Craddockc]

Jon,

Thank you for the reply. It appears as though my logic was faulty in thinking the auto qos voip was a complete command. It is not. So the [Trust] keyword is used to tell the port to trust which ever values are cming through (CoS or DSCP) depending on the port operation? From what I am reading if you use the [cisco-phone] keyword then it will trust the CoS values automatically, but only if it s coming from a Cisco IP Phone correct? Thanks.

Chris.

[Jon Marshall · Accepted solution]

Chris

Yes, the trust value used for "auto qos voip trust" depends on the operation of the port as you say.

And yes when you use the "cisco-phone" option it will trust the values but as you say only if it detects a Cisco IP phone and it does this by using CDP. 

What is not entirely clear, at least to me, is exactly which markings the "cisco-phone" option trusts. There seems to be conflicting information but the configuration guide talk about DSCP markings so it may be those but i can't say for sure as i have limited experience in that area.

Jon