Technical data

101
L2TP Tunneling - Virtual Dial-Up
This document has four parts:
•The
Introduction
provides a general overview of L2TP tunneling.
•The
L2TP Concepts
section explains LNS, L2TP client, LAC, dial user, tunnels, and sessions.
•
Configuration
describes preliminary configuration steps and verification steps and lists commands associated
with the configuration of L2TP and PPP sessions.
•The
Sample Configurations
section provides two examples with step-by-step instructions: a simple L2TP
client configuration example and a complete LNS and L2TP client configuration example.
Introduction
L2TP (Layer 2 Tunneling Protocol) is used to forward a PPP link from a remote site to a corporate site across the
Internet, thus creating virtual paths called tunnels. Because tunneling involves encapsulating data, packets can be
transported across networks using different protocols. The advantages for tunneling the PPP protocol are listed
below:
• Different network protocols such as NetBEUI, IPX, and Appletalk can be transported through the Internet
using a tunnel. The protocol packets are encapsulated and routed across the network through the Internet.
• Tunnels provide a way to reduce costs and complexity associated with remote dial-up networking by using a
local ISP: users connect the remote site by dialing into their local ISP and let the Internet handle the long-
distance connections, thus avoiding long-distance phone charges.
• Tunneling PPP allows compression of data through the entire tunnel, which translates into greater throughput.
• By allowing encryption over the PPP link, L2TP contributes to more secure networks over the Internet.
• Remote users can access the company network, even if there is a company firewall (provided, of course, that
tunnels can come through the firewall).
Note:
This feature can interoperate with any vendor that supports L2TP - Draft II.
L2TP Concepts
This section defines the major L2TP concepts such as LNS, L2TP client, LAC, and Dial user. These concepts are
illustrated with L2TP client examples. Also described are tunnels and sessions’ creations and destructions.
LNS, L2TP Client, LAC, and Dial User
An L2TP tunnel is created between an L2TP client and LNS. The L2TP client and LNS control the tunnel
using the L2TP protocol.
Since routers are more often configured as L2TP clients or LNS than as LACs, this document, therefore,
emphasizes L2TP client- and LNS-related information.