Introduction To L2Tp - 3Com MSR 50 Series Configuration Manual

1602 C 82: L2TP C
HAPTER

Introduction to L2TP

ONFIGURATION
(usually, Windows 2000 platform), however, limits the platforms that users can
use.
In general, a router or a VPN proprietary server is used as a gateway. VPDN
tunneling protocols are broken into three types: PPTP, L2F, and L2TP, of which L2TP
is widely used at present.
Background
The point-to-point protocol (PPP) defines an encapsulation mechanism for
transporting multi-protocol packets over Layer 2 (L2) point-to-point links. Typically,
a user obtains an L2 connection to a network access server (NAS) using
technologies such as dialup plain old telephone service (dial-up POTS), ISDN or
asymmetric digital subscriber line (ADSL). In such a configuration, PPP is used and
the L2 termination point and the PPP session endpoint reside on the same physical
device, namely the NAS.
L2TP tunnels PPP packets. It extends the PPP model by allowing the L2 and PPP
endpoints to reside on different devices interconnected over a packet-switched
network. By combining the best features of L2F and PPTP, L2TP becomes the Layer
2 tunneling industry standard defined by the Internet Engineering Task Force
(IETF).
Typical application
Figure 447 shows a typical VPDN built by using l2TP.
Figure 447 VPDN built by using L2TP
Remote user
PSDN/ISDN
Remote
embranchment
As shown in Figure
components: L2TP access concentrator (LAC) and L2TP network server (LNS).
An LAC, attached to a packet-switched network, has a PPP end system and the
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L2TP handling capability. An LAC is usually a network access server (NAS),
which provides access services to users over PSTN or ISDN networks.
An L2TP network server (LNS) is a device handling the server side of the L2TP
■
protocol on the PPP end system.
An LAC lies between LNSs and remote systems (remote users or remote branches).
It encapsulates the packets received from a remote system by using L2TP and
sends them to the LNS, and decapsulates the packets received from the LNS and
sends them to the remote system. Between an LAC and a remote system is a local
connection or PPP link. Usually, a PPP link is used in a VPDN application.
LAC
Internet
L2TP tunnel
447, a VPDN built by using L2TP includes two key
LNS
Inner server