3Com 3C13636 Configuration Manual page 1209

3Com Router 3000 Ethernet Family
Configuration Guide
between users and NAS, with endpoint of layer 2 link and PPP session sticking on the
same hardware.
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.
II. Typical L2TP networking application
Figure 2-1 shows a typical network where VPDN is constructed using L2TP:
Remote users
PC
Remote users
Figure 2-1 Network diagram of 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.
III. Technology details of L2TP protocol
1) Architecture of L2TP protocol
LAC
Internet
backbone network
PSTN/ISDN
L2TP channel
Access server
3Com Corporation
Chapter 2 Configuration of L2TP
LNS
Internal server
2-2