Product specifications

Efficient Networks
®
Router family
Command Line Interface Guide
Chapter 6: Connection Management
Efficient Networks
®
Page 6-27
L2TP Concepts
This section defines the major L2TP concepts and illustrates them with L2TP client
examples. It also describes the creation and destruction of tunnels and sessions.
Definitions
An L2TP tunnel is created between an L2TP client and an L2TP network server
(LNS). The client and server control the tunnel using the L2TP protocol.
As shown in the Figure 6-1, an L2TP client is used to tunnel a PPP session between a
small office (our router) and a corporate office through the Internet.
L2TP Client Illustration
The tunnel uses UDP/IP traffic as the transport medium over IP. This implementation
of L2TP as illustrated below shows a tunnel from a remote user’s perspective.
NOTE:
There is one PPP session over ISDN and another PPP session over the tunnel.
LNS and L2TP Client Relationship
The LNS acts as the supervising system. The L2TP client acts both as the dial user
and the LAC.
One end of the tunnel terminates at the L2TP client. The other end of the tunnel
terminates at the LNS.
One end of the PPP session going through the tunnel terminates at the L2TP client
acting as the dial user; the other end terminates at the LNS.
L2TP Network Server (LNS)
Point where the call is actually managed and terminated (e.g., within
a corporate network).
L2TP Access Concentrator (LAC)
Physical hardware (such as a router) used for placing and receiving
phone calls.
Dial User
The remote system or router that is either placing the call to the LAC
or receiving the call from the LAC. The dial user does not actually dial
in to the LNS or receive a call from the LNS, since this is a virtual con-
nection. The dial user is one end of a PPP session. The LNS is the
other end of the PPP session.
L2TP Client
The dial user and LAC combined in the same hardware device. In this
case, the PPP session is between the LAC and the LNS.