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Prestige 128+ ISDN Router
L2TP 10-1
Chapter 10
L2TP Support
This chapter shows how to reduce the cost of remote dial-up networking by
taking advantage of the Internet using L2TP (Layer 2 Tunneling Protocol).
10.1 What is L2TP?
Tunneling is the key to L2TP (and other virtual dial-up services). With tunneling,
protocol packets of one type of network are put inside or encapsulated in the
protocol packets of another network for transport across that network. A tunnel
has an entry point and an exit point that are essentially interfaces between two
different types of networks, although they are defined in software.
Dial-up users typically use the PPP (Point-to-Point Protocol) for an Internet
connection. PPP is a layer 2 protocol that frames data so it can be sent across a
dial-up connection. The protocol allows users to run TCP/IP software such as
Web browsers as if they were directly connected to the Internet. In fact, user
TCP/IP packets are put into PPP frames for transport across the dial-up link to an
ISP. The ISP then extracts the TCP/IP packets and forwards them on the Internet.
L2TP enhances PPP by granting a means for a remote user to extend a PPP link
across the Internet all the way to a corporate site. In essence, a tunnel is
established across the Internet from the ISP to a corporate site and frames are
transmitted through the tunnel. Once the tunnel is set up, the ISP is essentially
out of the picture and the user communicates to the corporate network over what
appears to be a direct dial-up connection.
L2TP authenticates the endpoints but does not encrypt the packets as they travel
across the Internet.