Layer 2 Forwarding (L2F) is a tunneling protocol developed by Cisco that is now considered obsolete. It was a precursor to the Layer 2 Tunneling Protocol (L2TP), which was created by the Internet Engineering Task Force (IETF) to merge the best features of L2F and Microsoft's Point-to-Point Tunneling Protocol (PPTP). Because L2TP became the industry standard, L2F is no longer developed or used in modern network environments, making it the least likely protocol on this list to be used for creating a VPN today.

A. L2TP is still used, typically in conjunction with IPSec for encryption, and is supported by many modern operating systems and network devices.
B. PPTP, while heavily deprecated due to significant security vulnerabilities, was widely adopted and may still be found in legacy systems or specific use cases.
C. IPSec is a secure, robust, and widely implemented protocol suite that forms the basis for most modern, standards-based VPNs.

1. Townsley

W.

Valencia

A.

Rubens

A.

Pall

G.

Zorn

G.

& Palter

B. (1999). RFC 2661: Layer Two Tunneling Protocol "L2TP". IETF. Section 1.1

"Introduction and Protocol Overview

" states

"L2TP represents a synthesis of the best features of two earlier tunneling protocols: Cisco's Layer 2 Forwarding (L2F) and Microsoft's Point-to-Point Tunneling Protocol (PPTP)." This document establishes L2TP as the standardized successor to L2F. Available at: https://doi.org/10.17487/RFC2661

2. Kaufman

C.

Perlman

R.

& Speciner

M. (2002). Network Security: Private Communication in a Public World (2nd ed.). Prentice Hall. In Chapter 18

"Tunneling (VPNs)

" the text discusses the history of VPN protocols

noting that L2F and PPTP were competing proprietary protocols that were ultimately superseded by the IETF standard L2TP.

3. Goralski

W. (2017). The Illustrated Network: How TCP/IP Works in a Modern Network (2nd ed.). Morgan Kaufmann. Chapter 15

"Virtual Private Networks (VPNs)

" describes L2F as a "Cisco-proprietary" protocol that was "combined with PPTP to form L2TP

" highlighting its replacement and subsequent obsolescence.