Layer 2 Forwarding Protocol

L2F, or Layer 2 Forwarding, is a tunneling protocol developed by Cisco Systems, Inc. to establish virtual private network connections over the Internet. L2F does not provide encryption or confidentiality by itself; It relies on the protocol being tunneled to provide privacy. L2F was specifically designed to tunnel Point-to-Point Protocol (PPP) traffic.^[1]

Use

Virtual dial-up allows many separate and autonomous protocol domains to share common access infrastructure including modems, Access Servers, and ISDN routers. RFCs prior to 2341 have specified protocols for supporting IP dial-up via SLIP and multiprotocol dial-up via PPP.

L2F packet structure

Bits 0-12													13-15			16-23								24-31
F	K	P	S	0	0	0	0	0	0	0	0	C	Ver			Protocol								Sequence (opt)
Multiplex ID																Client ID
Length																Payload offset (opt)
Packet key (optional)
Payload
																L2F Checksum (opt)

Other VPN protocols

• IPsec
• L2TP Layer 2 Tunneling Protocol
• PPTP Point-to-Point Tunneling Protocol