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.
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) |