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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.


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)
  L2F Checksum (opt)

Other VPN protocols

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

Source: Wikipedia