Hi Victor,

Thanks for your reply. I understand the concept of trusting dscp. My concern here is if I use this command "mls qos trust dscp" along with the service policy input command what would be the effect of it? Since my service policy is already classifying and marking dscp value I do not think it has any effect. I would like an expert opinion on this. Thanks much!!!

Fawad:

You do deserve an expert opinion, but unfortunately, I am no expert in QoS. There are some pretty brilliant and experienced individuals on here, like Jon Marshall, Edison Ortiz, Rick Burts, Joseph Doherty, etc. and a few more....so you definitely would want to wait for their inputs.

I will say, though, that from my understanding of QoS architecture, and from my experience with the few QoS deployments I have been responsible for, the mls qos trust dscp command is used for the very purpose of NOT changing the DSCP markings of the packets received on an interface, whether they were set by an application or by a policy applied by the administrator. It is meant to make the passing of traffic -- from a QoS perspective -- transparent.

So, I do agree with your assessment that the application of the mls qos trust dscp command when used with the service policy input command will not adversely effect your deployment or cause any unexpected results. In fact, I do believe that they are both necessary for a proper implementation. The purpose of one is to apply a QoS policy while the purpose of the other is to recognize it and not change it.

HTH

Victor

Applying both commands to the SAME interface?

OK.

Fawad:

I answered your question as though you were going to apply the service policy command on the interface that will apply the QoS policy and the mls qos trust dscp command on the intervening hops between source and destination, which is typically what you do.

Anyway, the concept remains: the "trust" command does just that - trusts. And the dscp markings remain unchanged.

Victor