That is even new for me.
I did a search and found that, now a days you no longer have to provide the Trust DSCP command, it is by default trusted.
Went through this White Paper and excerpts are below:
http://www.cisco.com/c/en/us/products/collateral/switches/catalyst-4500-series-switches/white_paper_c11-539588.html
The answer to your question comes from the following excerpt :-
"Previously supervisor engines relied on “port trust” to classify traffic; however, this does not fall into the MQC CLI construct. MQC provides a more flexible capability, i.e. all traffic is trusted by default, an administrator can change this trust state using a policy map. Another difference is the “internal DSCP” value used within the switch to place packets in the proper queue.
Cisco Catalyst 4500E Supervisor Engines do not use “internal DSCP”; rather, it relies on explicit matching of QoS values using class maps so that packets can be placed in the correct queue.
Also, note that there is no specific priority queue: it is not queue 3 or queue 1. The priority queue is simply configured within a class; therefore, it is not tied to a specific queue. One final difference is that of classification. Cisco Catalyst 4500E Supervisor Engines provide sequential classification rather than parallel. This allows the network administrator to classify traffic at egress based on the ingress markings. These markings can be done unconditionally, using a policer or using a table map. Based on these changes, QoS CLI will now be more contiguous on the Supervisor Engines as it will now have standard Cisco MQC CLI, making configuration management much simpler"
HTH,
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Regards
