Traffic classes are categories of traffic (packets) that are grouped on the basis of similarity of traffic are called class maps. Classifying network traffic allows you to enable a quality of service (QoS) strategy in your network.
Figure 1. Criteria for Classifying Network Traffic
Identifying and categorizing network traffic into traffic classes (that is, classifying packets) enables you to handle different types of traffic by separating network traffic into different categories.
Classifying network traffic allows you to see the kinds of traffic you have and treat some types of traffic differently than others. Identifying and organizing network traffic allows you to allocate network resources to deliver the best performance for each type of traffic.
You can place network traffic with a specific IP precedence into one traffic class, while you place traffic with a specific differentiated services code point (DSCP) value into another traffic class. Each traffic class can be given a different QoS treatment, which you configure in a policy map later.
You define each class of traffic in a class map based upon criteria, such as the IP precedence or Class of Service (CoS). The allowable criteria for mapping classes of traffic is listed in Criteria for Mapping Classes section. You can match the criteria to your traffic as follows:
Matching or not matching one
Matching or not matching multiple
Matching or not matching another class map
Some of the criteria used in traffic class maps relates only to one direction of traffic—either ingress or egress. For example, the internal label QoS group has no meaning on ingress traffic because it has not yet been assigned a value.
Traffic that fails to match any traffic class in a QoS policy map is assigned to a default class of traffic. called class-default. The class-default can be referenced in a QoS policy map to select this unmatched traffic.