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Trust dscp on 4500 Sup 6E


I was configuring QoS for Telephony recently at sites with 6509E core and 4510R-E with Sup 6E access switches. The plan was to add a policy map to the access ports to set dscp values and simply trust on all up/downlinks, standard stuff. All was applied successfully until trying to configure "qos trust dscp" on the 10G links into the core on the 4500s. These links used the 10G ports on the Sup 6E itself not a 10G line card.

The command was rejected and a closer look at the command ref for 12.2(53)SG states "This command is not supported on the Supervisor Engine 6-E and Catalyst 4900M chassis."

I tried to add MQC as a workaround with a "trust dscp" command within a policy map. This too was rejected. As QoS needs to be globally enabled on these devices to mark packets at the access layer the default for ports becomes dscp untrust.

I thought an IOS upgrade might be the answer but the command ref for the latest version 12.2(54)SG also lists this command as unsupported.

My question of course is what's the solution?

The current workaround is applying the access layer MQC to the uplink ports to re-set the dscp markings. This is undesirable however. Does anyone know how to set these ports to trust received dscp markings?



Answered my own question I think by reading the following white paper

"QoS does not need to be enabled on the Supervisor Engine 6-E, it is on by default in compliance with the MQC construct.
When a packet arrives at an interface, there are two options to take into consideration: is there a policy attached or
not? If the packet arrives with or without a marking and there is not a policy attached to the interface, packets will
flow through the switch untouched. There are no questions as to where the packet came from or if it has a valid marking.
If the packet arrives with or without a marking, and a policy is attached to the interface, the packet will only
then be subject to the policy classification."

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