Hello Samer
Class 4 has the highest priority, so if you have AF33, it will have a lower drop probability than AF21 for example. But within the same class, the higher the number the higher the drop probability, so AF13 will more likely be dropped compared to AF11. So yes, you are correct.
AF31 is in a higher priority class than AF21, so AF 21 has the higher drop probability.
EF are also considered part of the DSCP based WREN procedure and are given an even higher priority than the AF markings. As for CS3 and CS4, they have a higher drop probability than the AF series of values.
Ultimately, when using the random-detect dscp-based command, you are telling the device to use the six bit DSCP value as the criteria for random drops. As Cisco documentation states, the “dscp-based argument enables WRED to use the DSCP value of a packet when calculating drop probability.” This means that the whole value (6 bits) is used, which means that CS, AF and EF values are considered in this calculation.
You are also able to optionally specify the minimum and maximum packet thresholds for the DSCP value using the random-detect dscp command.
More info can be found here:
The fair-queue command is used to implement Distributed WRED (DWRED). It’s a feature only available in the 7000 series routers. This specific command specifies the number of queues to be reserved for the specific traffic class. You can find out more about DWRED here:
I hope this has been helpful!
Laz