Specifications

124 Chapter 5. Configuring Software Options
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 an L2TP server.
An L2TP server can communicate simultaneously with more than one L2TP client.
An L2TP client can communicate simultaneously with more than one L2TP server.
Some L2TP implementations including the one discussed in this section allow the same router to act as both
an L2TP client and an L2TP server 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.
Sessions
Sessions can be thought of as switched virtual circuit ÒcallsÓ carried within a tunnel and can only exist within
tunnels. One session carries one ÒcallÓ. This ÒcallÓ is one PPP session. Multiple sessions can exist within a
tunnel. The following briefly discusses how sessions are created and destroyed.
¥ Session creation
Traffic destined to a remote entry (located at the end of the tunnel) initiates a tunnel session. When the
L2TP client wishes to establish a session to an LNS, the L2TP client assumes the role of a LAC and
sends control packets containing incoming call information to the LNS over the tunnel.
¥ Session destruction
A tunnel session automatically times out after the data session stops. When instructed to destroy a
session, the L2TP client closes any PPP session associated with that session. The L2TP client may also
send control messages to the LNS indicating that the L2TP client wishes to end the PPP session.
When the LNS wants to hang up the call, it sends control messages destroying the session.
Configuration
Preliminary Steps to Configure a Tunnel
The following logical steps should be considered before configuring a tunnel:
1. Decide if the router should act as an L2TP Client or LNS.