Operation Manual

Chapter 6
L2TP
L2TP Introduction
L2TP is a real tunneling protocol, much more elaborate protocol than other
tunneling protocols i.e. GRE. There are multiple levels of negotiation at the
connection setup time – a tunnel is negotiated, and then a call is negotiated
within the tunnel, and then a PPP link is negotiated within the call.
Basically, the purpose of L2TP is to tunnel a PPP link across the Internet. So,
a remote user can have a PPP connection to a central site, or two offices can
have a PPP connection to each other. This PPP connection, although across
the Internet, can be treated as though it were a direct connection over a
leased line, and all the richness of the PPP protocol can be used. This
provides a simple model for the setup at each end – all the complexity is
confined to the protocol implementation in the end-point routers.
The AT-RG213 provides the tunneling of PPP packets across an IP Network
in a transparent way to both end-points.
The AT-RG213 acts as an L2TP Access Concentrator (LAC) creating a tunnel
across the IP infrastructure from itself (LAC) and the L2TP Network Server
(LNS), enabling Point-to-Point Protocol layer frames to be encapsulated and
carried across the Internet. Figure 17 shows the related network model.