3Com Switch 8800 Family IPsec Module Configuration and Command reference Guide

104 CHAPTER 8: CONFIGURATION OF L2TP
L2TP provides Tunnel transmission for PPP link layer packets. It extents PPP model
in that it permits link endpoint of layer 2 and PPP session point staying at different
devices and allows information interaction by using packet switching network
technologies. It combines the advantages of PPTP and L2F. Therefore, it becomes
the industrial standard of IETF in layer 2 Tunneling.
Typical L2TP networking application
Figure 22 shows a typical network where VPDN is constructed using L2TP:
Figure 22 Network diagram for typical VPDN application created by L2TP
In this figure, LAC stands for L2TP Access Concentrator, a switching network
device with the capability to process PPP and L2TP requests. Usually, LAC functions
as Network Access Server (NAS) to provide access service to users by making use
of PSTN/ISDN. LNS stands for L2TP Network Server, a device functioning in the PPP
system as L2TP server.
LAC lies between LNS and remote system (remote users and remote branches) to
transmit packets between them, encapsulate packets from remote system in L2TP
protocol and send the encapsulated packets to LNS, and decapsulate packets from
LNS and send the remaining part to remote system. Local connection or PPP link
can be adopted between LAC and remote system, but PPP link is always involved
in VPDN applications. As one end of the L2TP Tunnel, LNS is the peer device of
LAC, and also is the logic terminating point of PPP session transmitted in Tunnel by
LAC.
Technology details of L2TP protocol
1 Architecture of L2TP protocol
Figure 23 Architecture of L2TP protocol
The architecture of L2TP protocol shown above describes the relationship between
PPP frame, control Tunnel and data Tunnel. PPP frame is transmitted in unreliable
PC
PSTN/ISDN
Remote users
NAS
Remote users
Internal server
Internet
backbone network
L2TP
channel
LAC
LNS
PC
PSTN/ISDN
Remote users
NAS
Remote users
Internal server
Internet
backbone network
L2TP
channel
LAC
LNS
Packet transmission packet (UDP,ĂĂ)
L2TP Data message
(unreliable)
L2TP Data message
PPP Frame
L2TP Control tunnel
(reliable)
L2TP Control message