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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Supported LAC Service Configurations for the GGSN and P-GW
As mentioned previously, L2TP is supported through the configuration of LAC services on the system. Each LAC
service is bound to a single system interface configured within the same system destination context as displayed in
following figure.
Figure 40.
GGSN LAC Service Configuration
LAC services are applied to incoming subscriber PDP contexts based on the configuration of attributes either in the
GGSN/’s Access Point Name (APN) templates or in the subscriber’s profile. Subscriber profiles can be configured
locally on the system or remotely on a RADIUS server.
LAC service also supports domain-based L2TP tunneling with LNS. This method is used to create multiple tunnels
between LAC and LNS on the basis of values received in “Tunnel-Client-Auth-ID” or “Tunnel-Server-Auth-ID”
attribute received from AAA Server in Access-Accept as a key for tunnel selection and creation. When the LAC needs
to establish a new L2TP session, it first checks if there is any existing L2TP tunnel with the peer LNS based on the
value of key “Tunnel-Client-Auth-ID” or “Tunnel-Server-Auth-ID” attribute. If no such tunnel exists for the key, it will
create a new Tunnel with the LNS.
If LAC service needs to establish a new tunnel for new L2TP session with LNS and the tunnel create request fails
because maximum tunnel creation limit is reached, LAC will try other LNS addresses received from AAA server in
Access-Accept message. If all available peer-LNS are exhausted, LAC service will reject the call
