L2TP Access Concentrator
Supported LAC Service Configurations for the GGSN and P-GW ▀
Cisco ASR 5x00 Packet Data Network Gateway Administration Guide ▄
391
L2TP tunnel parameters are configured within the APN template and are applied to all subscribers accessing the APN.
However, L2TP operation will differ depending on the subscriber’s PDP context type as described below:
Transparent IP:
The APN template’s L2TP parameter settings will be applied to the session.
Non-transparent IP:
Since authentication is required, L2TP parameter attributes in the subscriber profile (if
configured) will take precedence over the settings in the APN template.
PPP:
The APN template’s L2TP parameter settings will be applied and all of the subscriber’s PPP packets will
be forwarded to the specified LNS.
More detailed information is located in the sections that follow.
Transparent IP PDP Context Processing with L2TP Support
The following figure and the text that follows describe how transparent IP PDP contexts are processed when L2TP
tunneling is enabled.
Figure 41.
Transparent IP PDP Context Call Processing with L2TP Tunneling
1.
A Create PDP Context Request message for a subscriber session is sent from the SGSN to the GGSN service over the Gn
interface. The message contains information such as the PDP Type, APN, and charging characteristics.
2.
The GGSN determines whether or not it is configured with an APN identical to the one specified in the message. If so, it
determines how to process the session based on the configuration of the APN.
The APN configuration indicates such things as the IP address of the LNS, the system destination context in which a LAC
service is configured, and the outbound username and password that will be used by the LNS to authenticate incoming
