ISG traffic classes provide differentiated behavior for different traffic streams to and from a particular subscriber. Each traffic stream is represented by a classification and a set of applied features. A traffic class, also known as a flow, is a kind of service.
For traffic to be classified into streams, you must specify an access control list (ACL) that classifies the traffic and the direction of the traffic to which the ACL applies (inbound or outbound). Optionally, the priority of the traffic class can also be specified. Traffic that meets the specifications of a traffic class is said to
match the traffic class. Once a match is made, features defined in the traffic policy are executed for that traffic class.
The priority of a traffic class determines which class is used first for a specified match if more than one traffic policy has been activated for a single session. In other words, if a packet matches more than one traffic class, it is classified to the class with the higher priority.
Packets that do not match any of the ACLs are considered part of the default traffic class and are processed as if a traffic policy was not applied to the session. A default class exists for every service. The default action of the default class is to pass traffic, or the default class can be configured to drop traffic. Default traffic is accounted for in the main session accounting. A service can contain one traffic class and one default class.
ISG traffic classes are created dynamically, either at session start or later during the life of the session, when a service with a classification (the class definition of the service contains at least one named or numbered ACL) is applied to a session. A service with a classification is called a flow service. A service without a classification is called a classless service.
Traffic classes are assigned unique identifiers that can be tracked with Cisco IOS