Policy maps prioritize network traffic by class. You create policy maps to define how to treat each class of traffic so that it is prioritized for the best quality of service.
Marking is the process of marking packets, that is, changing one of the following in the packet for QoS purposes:
You can map a traffic class to a DSCP, which is an indicator of the service level for a specified frame. The DSCP value ranges from 0 to 63, and the default is 0. A DSCP value of 46 is disallowed.
Service policies are specified using policy maps. Policy maps provide an ordered mapping of class maps to service levels. You can specify multiple class maps within a policy map, and map a class map to a high, medium, or low service level. The default priority is low. The policy map name is restricted to 63 alphanumeric characters.
The order of the class maps within a policy map is important to determine the order in which the frame is compared to class maps. The first matching class map has the corresponding priority marked in the frame.
Figure 1. Packet Fields Available for Marking
Marking is the setting of QoS information that is related to a packet. You can set the value of standardt QoS fields IP precedence, DSCP and Class of Service (CoS), and internal labels that can be used in subsequent actions such as policing.
Once your traffic classes are defined, you can reference them in the policy map where you also define how they should be marked. We recommend that you keep the policy simple by using no more than four classes.