User guide

Chapter 4. Configuring Special Features 107
LNS, L2TP Client, LAC, and Dial User
An L2TP tunnel is created between an L2TP client and LNS. The L2TP client and LNS control the tunnel
using the L2TP protocol.
Since routers are more often configured as L2TP clients or LNS than as LACs, this section, therefore,
emphasizes L2TP client- and LNS-related information.
¥ LNS (L2TP Network Server)
The LNS is the point where the call is actually managed and terminated (e.g., within a corporate
network).
¥ L2TP Client
With an L2TP client, the dial user and LAC are combined in the same hardware device. In this case, the
PPP session is between the LAC and the LNS.
As shown in the following illustration (Figure 1), an L2TP client is used to tunnel a PPP session between
a small office (our router) and a corporate office through the Internet.
¥ LAC (L2TP Access Concentrator)
The LAC can be envisioned as the physical hardware (e.g., a router) used for placing and receiving
phone calls.
¥ Dial User
A dial user is 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
connection.
The dial user is one end of a PPP session. The LNS is the other end of the PPP session.
L2TP Client Example
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 userÕs perspective.
Note: There is one PPP session over ISDN and another PPP session over the tunnel.