Cisco ASA uses three methods to achieve QoS, namely traffic supervision, traffic shaping and priority queuing. The difference between the three methods is that when the traffic reaches the set threshold, the traffic control discards the packet, and traffic shaping puts the packet in the waiting queue (only supported after version 7.2.4). The priority queue is not affected by the first two, and its priority is higher than the first two. In the same situation, the traffic using the priority queue passes through first. In practical work, these three methods are mixed. Combined with the figure below, from bottom to top, I briefly describe the steps of setting QoS:

1) Set the class mapping to match the traffic you want to control. You can use ACL, dscp, tunnel group …

2) Set policy mapping, and apply attributes or (and) policies on the corresponding category mapping.

3) Set the service policy to match the required policy mapping.

4) Apply service policy to the interface.