How The System Selects Contexts; Context Selection For Subscriber Sessions - Cisco ASR 5000 Series Administration Manual

Understanding the Service Operation

How the System Selects Contexts

This section provides details about the process that is used to determine which context to use for context-level
administrative user and/or subscriber sessions. Understanding this process allows you to better plan your configuration
in terms of how many contexts and interfaces need to be configured.

Context Selection for Subscriber Sessions

The context selection process for a subscriber session is more involved than that for the administrative users.
The source context used to service a subscriber session is the same as the context in which the GGSN service is
configured. Each GGSN service is bound to an IP address in a source context. The SGSNs select which IP address to
use, typically by using DNS. Once a subscriber has established a PDP context with a GGSN, the SGSNs continue to use
that same PDP context and GGSN as the subscriber moves about the network.
Destination contexts are selected based on APN configuration. When the system receives a Create PDP Context
Request message from the SGSN, it examines the APN that was provided. If the APN is not found on the system, the
system rejects the request.
After the APN has been found, the system may choose a different APN based on the system's virtual APN
configuration. In any event, a final APN is selected by the system
The system determines the destination context to use based on a parameter contained within the final APN
configuration. If a valid destination context name is configured for this parameter, it is used. If the name is not valid, or
if it is not configured, the system uses the context in which the APN is configured.
Once the system locates the context in which the APN is configured, it uses that context for subscriber authentication
and RADIUS-based accounting (if enabled). Any parameters returned by the RADIUS server during the subscriber
authentication/authorization override APN configuration parameters.
If GTPP-based accounting is enabled, the system uses the source context for accounting. That context may be
overridden by configuring a different accounting context to use in the GGSN service configuration.
OL-22944-02
Cisco ASR 5000 Series Gateway GPRS Support Node Administration Guide ▄
How the System Selects Contexts ▀