User guide

108 Chapter 4. Configuring Special Features
Figure 1
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.
Tunnels
Tunnels are virtual paths that exist between an L2TP client and LNS.
An LNS can communicate simultaneously with more than one L2TP client.
An L2TP client can communicate simultaneously with more than one LNS.
Some L2TP implementations including the one discussed in this section allow the same router to act as both
an L2TP client and LNS simultaneously, if so configured.
Caution: Verify that the IP address of the other end of the tunnel is correctly routed through the right, local
interface/remote and will not appear to be routed through the tunnel. An attempt to route the tunnel endpoint
within itself will fail.
INTERNET
L2TP Client:
Dial User+LAC
(ISDN router)
LNS Router
Remote User
Company
Company
LAN/server
PC
TUNNEL
Logical Link
Physical Link Physical Link
IP traffic to the Internet
PPP session
ISDN line
DSL/ATM traffic
PPP session running over the tunnel