A question about Cisco MLS and MQC QoS.

I read two things on a Cisco guide: https://www.cisco.com/c/en/us/support/docs/switches/catalyst-3850-series-switches/118629-technote-qos-00.html

On the 3750, by default the QoS is disabled whereas on the 3850, it is enabled. Also in order to preserve Layer 2 (L2)/Layer 3 (L3) QoS marking on the 3750 platform, a trust configuration must be applied.

In the case of the 3850, all packets by default are trusted (the L2/L3 QoS marking is preserved), unless you change it with an application of a specific policy map on the ingress or egress interface.

And, to perform a trust cos in a MQC model in Ingress:

Ingress: apply policy-map trust-cos Ingress policy:

3850#show run policy-map trust-cos 
class class-default
   set cos cos table default

3850#show table-map default
 Table Map default
    default copy

So the question, the guide explain that the trust cos/dscp is enabled by default on the MQC QoS model, but the guide explain how to apply a trust cos/dscp in ingress interface in MQC, so, what's true ?

1 Answer 1


I agree, that looks contradictory. For the DSCP case it says "Ingress: default trust dscp, no policy needed" but for the CoS case it says "Ingress: apply policy-map trust-cos".

You can ask Cisco to clarify their documentation, but rather than wait for a clarification, you can go ahead and implement an explicit input policy anyway. Configuring an explicitly defined policy is probably better than depending on some poorly documented implicit behaviour, and is also more portable across multiple devices/software releases.

If you want to trust the incoming CoS, use a table-map that maps 1 to 1, 2 to 2 and so on. This is what the example in the documentation ("trust-cos" policy map) does.

If you do not want to trust the incoming CoS, use a policy-map that sets the cos to 0 unconditionally.

policy-map no-trust-cos
 class class-default
  set cos default

(or set cos 0)

Similarly for trusting/not trusting DSCP.

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