Which of the following is LESS likely to be used today in creating a Virtual Private Network?

A.
L2TP

B.
PPTP

C.
IPSec

D.
L2F

Explanation:
Layer 2 Forwarding Protocol (L2F) is rarely used today.
The following are the three most common VPN communications protocol standards:
Point-to-Point Tunneling Protocol (PPTP). PPTP works at the Data Link Layer of the OSI model. Designed
for individual client to server connections, it enables only a single point-to-point connection per session. Thisstandard is very common with asynchronous connections that use Win9x or NT clients. PPTP uses native
Point-to-Point Protocol (PPP) authentication and encryption services.
Layer 2 Tunneling Protocol (L2TP). L2TP is a combination of PPTP and the earlier Layer 2 Forwarding
Protocol (L2F) that works at the Data Link Layer like PPTP. It has become an accepted tunneling standard
for VPNs. In fact, dial-up VPNs use this standard quite frequently. Like PPTP, this standard was designed
for single point-topoint client to server connections. Note that multiple protocols can be encapsulated within
the L2TP tunnel.
IPSec. IPSec operates at the Network Layer and it enables multiple and simultaneous tunnels, unlike the
single connection of the previous standards. IPSec has the functionality to encrypt and authenticate IP data.
It is built into the new IPv6 standard, and is used as an add-on to the current IPv4. While PPTP and L2TP
are aimed more at dial-up VPNs, IPSec focuses more on network-to-network connectivity.
Incorrect Answers:
A: L2TP and IPSec are commonly used together for VPNs today.
B: PPTP is not used as commonly as L2TP and IPSec but it is more common than L2F.
C: L2TP and IPSec are commonly used together for VPNs today.

Krutz, Ronald L. and Russel Dean Vines, The CISSP Prep Guide: Mastering the Ten Domains of Computer
Security, John Wiley & Sons, New York, 2001, p. 92