4.1 L2TP Overview
The L2TP protocol, which embodies the advantages of L2F and PPTP, is a industry standard on
Layer 2 tunnel protocols defined by the IETF.
4.1.1 Introduction to L2TP
L2TP messages are used in the maintenance of L2TP tunnels and transmission of PPP frames.
These messages are transmitted through UDP port 1701 in the TCP/IP protocol suite. L2TP uses
two types of messages: control messages and data messages.
The Point to Point Protocol (PPP) defines a kind of encapsulation technique, which ensures the
transmission of datagram of multiple protocols over Layer 2 point-to-point links. During the
transmission, PPP is performed between users and the Network Access Server (NAS), with Layer
2 link endpoints and PPP session termination points residing on the same device.
The Layer 2 Tunneling Protocol (L2TP) is used to transmit the Layer 2 PPP datagram over a
tunnel. L2TP extends the PPP model because L2TP permits Layer 2 link endpoints and PPP
session termination points to stay at different devices, and can realize information exchange
based on packet-switching technology. By combining the advantages of the Layer 2 Forwarding
(L2F) and Point-to-Point Tunneling Protocol (PPTP), L2TP is defined by the Internet
Engineering Task Force (IETF) as an industry standard of the Layer 2 tunneling protocol.
4.1.2 L2TP Features Supported by the AR1200
The AR1200 supports three L2TP tunnel modes.
Three Typical L2TP Tunnel Modes
Figure 4-1 shows the tunnel modes between the remote system and the L2TP Network Server
(LNS), and between the L2TP Access Concentrator (LAC) client (host running L2TP) and LNS.
Figure 4-1 Networking diagram of three typical L2TP tunnel modes
Remote
system
LAC
LAC
LNS
LAC
client
LNS
Internal
server
PSTN/
ISDN
Network
Internal
server
LAN
PC
Network
Huawei AR1200 Series Enterprise Routers
Configuration Guide - VPN 4 L2TP Configuration
Issue 01 (2012-04-20) Huawei Proprietary and Confidential
Copyright © Huawei Technologies Co., Ltd.
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