Layer 2 Forwarding Protocol
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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
|Multiplex ID||Client ID|
|Length||Payload offset (opt)|
|Packet key (optional)|
|L2F Checksum (opt)|