Cisco IOS XE, Configuration Manual, Page: 207 / 358

Configuring ISG Subscriber Services
Information About ISG Subscriber Services
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Primary Services
When a network-forwarding policy is included in a service profile or service policy map, the service is
known as a primary service. Primary services are mutually exclusive and may not be simultaneously
active. Upon activation of a new primary service, ISG will deactivate the existing primary service and
any other services dependent on the existing primary service through association with a service group.
If a primary service is deactivated, sessions may be left without a network-forwarding policy, that is,
with no means to route or forward packets. A policy may be applied to defend against this condition such
that a specific service is activated upon deactivation of all others (or all other primary services). This
backup service would return network-forwarding policy to the session and allow the subscriber to reach
a web portal. However, it should be noted that an IP session will not be automatically terminated when
all services are deactivated unless such a policy has been defined and applied.
Traffic Classes and Traffic Class Priority
In order for traffic to be classified into flows, the following information must be specified: an access
control list (ACL) that classifies the flow, and direction of 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 will be executed for that traffic class.
If there are multiple services with the traffic classes, by default packets are matched according to the
order in which the services are installed. Traffic classes can also be assigned priority. The priority of a
traffic class determines which class will be used first for a specified match. In other words, if a packet
matches more than one traffic class, it will be classified to the class with higher priority.
Packets that do not match any of the ACLs are considered to be part of default traffic and are processed
as if a traffic policy were not applied to the session. A default class exists for every service, and the
default action of the default class is to pass traffic. 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.
Traffic classes are assigned unique identifiers that can be tracked with Cisco IOS show commands.
Traffic Policies
Traffic policies define the handling of data packets. A traffic policy contains a traffic class and one or
more features. Whereas you can specify the event that will trigger an ISG control policy, the trigger for
a traffic policy is implicit—the arrival of a data packet.
The features configured within a traffic policy apply only to the traffic defined by the traffic class.
Multiple traffic policies with various features can be applied to a session.
ISG Features
An ISG feature is a functional component that performs a specific operation on a session’s data stream.
A feature may or may not be associated with a traffic class. However, once associated with a traffic class,
a feature can be applied only to the packets that match that traffic class. Otherwise, the feature is applied
to all packets for that session.