QoS mark stripping

Unanswered Question
Apr 21st, 2009


We are having trouble with our QoS setup:

We have two 4500 switches, A and B that are connected by a trunk link. The packet leaves Switch A with a mark of QoS 5 but the value returns to default after passing through the link. Why does this issue occur even though qos trust dscp is configured?


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Giuseppe Larosa Wed, 04/22/2009 - 02:23

Hello Greg,

is the link between the two switches a trunk ?

I wonder if you should also trust cos

Hope to help


greg.loria Wed, 04/22/2009 - 17:12

Hi Giuseppe,

Yes, the link is a trunk, a 100MB link provided by a telco. We were also wondering if we should trust cos. Is it safe to enable both the trust cos and trust dscp?

Thanks for the input.

Jon Marshall Thu, 04/23/2009 - 01:23


As far as i know you have to choose between trust CoS and trust DSCP. But on a trunk link you should be trusting CoS not DSCP anyway.

You can then determine which DSCP value is used by the switch by using the CoS-to-DSCP map.


greg.loria Thu, 04/23/2009 - 03:21

Ok Jon, we will try using trust COS instead. Oh, and is there a documentation showing that we should trust CoS on trunk links? This would be a great help also.

Thank you very much for the recommendation.

Best Regards.

davy.timmermans Wed, 04/22/2009 - 02:55

QoS is enabled on both switches?

trust DSCP is configured at each end of the trunk?

Are you checking with an outbound and inbound policy map on the trunk interface to see for matches on QoS5

You have to check if it doesn't pass any untrusted interface because an unconfigured interface will remap it to 0 (if QoS is globally enabled)


greg.loria Wed, 04/22/2009 - 17:19

Hi Davy,

Thanks for the thought.

Yes, trust DSCP is configured on both switches.

We sniffed the packet to check the QoS mark. As it passed through the link and enters the other switch, the mark goes back to default.

So far, it doesn't pass to any untrusted interfaces. The problem just occurs with this link. Any more thoughts on this matter?

Kind Regards.

Giuseppe Larosa Thu, 04/23/2009 - 01:32

Hello Greg,

I agree with Jon try to trust cos instead of trusting DSCP and kindly report if this makes any difference

Hope to help


greg.loria Thu, 04/23/2009 - 03:22

Hi Giuseppe,

Yes, I agree with you two. We will try to use trust cos.



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