Played around with a SMC SMCGS26C-Smart switch testing the DSCP tagging functionality prior to implementing. From what I have found you need to enable DSCP Egress rewriting for this to occur once it has been tagged into the queue.
So, the basics (overview) from what I have learned
Advanced > QOS > Port Classification defines the default for the port in terms of how it classifies packets coming into the switch.
As can be seen by this picture, it is configurable via each port. Basically, you are determining what default queue, drop priority is used for a packet that is untagged. If DSCP based is ticked, then packets are checked for DSCP values and mapped to desired class and checked to see if they are trusted (configure via Advanced>QoS > DSCP-Based QoS)
QoS class of 0 has the lowest priority
Advanced > QoS > Port Policing is used to limit the amount of traffic entering the port. Packets that exceed the rate defined are dropped.
Advanced > QoS > Port Scheduler shows show the port traffic is monitored and queued.
Strict priority requires that all traffic in a higher priority queue is processed before a lower priority queue. Deficit Weighted Round-Robin specifies a scheduling weight for each queue. Note that the schedule priority only shows 6 queues? This is because queue 6 and 7 or strict mode.
Port scheduling and Port Shaping are two views into the same information. Port Schedulars show how the scheduling is done for each port in terms of servicing the queues of the port.
Port Shaping shows bandwidth limitations that may be being imposed to the queues.
Advanced > QoS > DSCP-Based QoS allows the administrator to determine which DSCP values arriving at the switch are to be trusted, and to which queue the traffic should be put into inside the switch.
Advanced > QoS > DSCP Control List is where classification of packets into queues is performed. Create rules to classify packets into correct queues and set DSCP values as required.
Now for the crutch of the matter:
To set DSCP values on a packet, configure DSCP Control List to classify packets into the queues that you want to use. Additionally, set the DSCP value for the outgoing packet.
On the Port DSCP page, set the Egress Rewrite to an enable setting for the DSCP value of the packet to be written in outgoing packets. Failure to do so WILL result in packets not being tagged with DSCP values from the queues.
Point to note: When is a port mirror not a port mirror?
A. When it is an SMC!! If you enable port mirroring on the switch (locally), ensure that you have Egress DSCP rewriting enabled otherwise it will not have the DSCP value in the packet.