QoS DSCP AF Class Comparisons

Question

I'm having trouble wrapping my head around DSCP AF Classes and what--all else being equal--gets dropped first.

I understand packets are given a Class, 1-4. The higher the Class number, the higher priority that packet gets. I must assume that higher priority means lower chance of being dropped.

Is this assumption correct: Higher Class = Higher Priority = Lower chance of being dropped?

I also understand that packets are given a drop precedence, 1-3... and in this case the lower the drop precedence, the lower the chance of being dropped.

During congestion, does higher Class traffic have precedence over lower Class traffic, regardless of Drop Priority?

If so, this leads to the absolutely counter-intuitive scenario where, while AF32 is less likely to be dropped than AF13 (32 > 13), AF32 is more likely to be dropped than AF31 (despite 32 > 31)... and that's really throwing me for a loop!

Answer 1

I understand packets are given a Class, 1-4. The higher the Class number, the higher priority that packet gets. I must assume that higher priority means lower chance of being dropped.

Is this assumption correct: Higher Class = Higher Priority = Lower chance of being dropped?

During congestion, does higher Class traffic have precedence over lower Class traffic, regardless of Drop Priority?

Both of these are incorrect. Each class has a minimum amount of resources (e.g., bandwidth) allocated to it. Different classes will have different amounts. So for example, Class 4 may have 30% of the link bandwidth allocated to it, while class 3 has only 10%. That means during congestion, class 4 is guaranteed a minimum of 30% of the link bandwidth.

Note that class 4 doesn't have higher priority than 3, it just means it has a different amount of resources available to it.

Within each class, the drop priority indicates the probability that the traffic would be dropped.