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. And see here for current known issues.

New Member

question of QoS trust behavior

I have a question of QoS trust behavior, I am not sure if what I remember is correct.

If I config mls qos trust cos, what will be done for dscp? will the dscp be remarked with the config of "mls qos map cos-dscp"? and for mls qos trust dscp, will the cos be remarked with the config of "mls qos map dscp-cos"?

And for default, if no mls qos trust xxx command, all cos and dscp will be reseted to 0?

Hope somebody can give me some idea, thanks!

Best Regards,

Teru Lei


Re: question of QoS trust behavior

Your memory is not as bad as you feared. You have given a fairly adequate description of how it actually works.



Re: question of QoS trust behavior

Hi Lei,

You are very near.

Here's my attempt:

Ip phone marks voice trafic with COS 5, IP Precedence 5 and DSCP EF.

IP phone marks signalling traffic as COS 3, Precedence 3 & DSCP AF31.

If the PC connected to phone marks the pkts with any COS value, then IP phone rewrite the PC traffic with COS 0, Precedence 0 & DSCP default.

The traffic is now passed to switch. If the switch doesnt have any "mls qos trust" on ports, then packets are forwarded with marking removed. however, trust command helps the switch to retain the markings. If the switch is L2, it forwards the traffic based on COS values. But if it is L3, then Precedence or DSCP values are considered.

Since COS values are used only within a LAN, when the pkts reach the router for outside networks they have to be marked again with Precedence or DSCP, based on the incoming COS values.

hope this clarified the doubt.

pls rate if helped.

New Member

Re: question of QoS trust behavior

Thanks All!