L2TP Access Concentrator
Supported LAC Service Configurations for the GGSN and P-GW ▀
Cisco ASR 5x00 Packet Data Network Gateway Administration Guide ▄
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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 sessions. If no outbound information is configured, the subscriber’s username is sent to
the peer LNS.
3.
The GGSN service authenticates the subscriber. The subscriber could be configured either locally or remotely on
a RADIUS server. Figure above shows subscriber authentication using a RADIUS AAA server.
As part of the authentication, the RADIUS server returns an Access-Accept message.
The message may include attributes indicating that session data is to be tunneled using L2TP, and the name and
location of the LAC service to use. An attribute could also be provided indicating the LNS peer to connect to.
If these attributes are supplied, they take precedence over those specified in the APN template.
4.
The GGSN returns an affirmative Create PDP Context Response to the SGSN over the Gn interface.
5.
The GGSN passes data received from the MS to a LAC service.
6.
The LAC service encapsulates the IP packets and forwards it to the appropriate Gi interface for delivery to the
LNS.
7.
The LNS un-encapsulates the packets and processes them as needed.The processing includes authentication and
IP address allocation.
PPP PDP Context Processing with L2TP Support
The following figure and the text that follows describe how non-transparent IP PDP contexts are processed when L2TP
tunneling is enabled.
Figure 43.
PPP PDP Context Call Processing with L2TP Tunneling
