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L2F: Layer 2 Forwarding Protocol

The Layer 2 Forward protocol (L2F) is used to establish a secure tunnel across a public infrastructure (such as the Internet) that connects an ISP POP to a enterprise home gateway. This tunnel creates a virtual point-to-point connection between the user and the enterprise customer's network.

Layer Two Forwarding protocol (L2F) permits the tunneling of the link layer (i.e., HDLC , async HDLC, or SLIP frames) of higher level protocols. Using such tunnels, it is possible to divorce the location of the initial dial-up server from the location at which the dial-up protocol connection is terminated and access to the network provided.

L2F allows encapsulation of PPP /SLIP packets within L2F. The ISP NAS and the Home gateway require a common understanding of the encapsulation protocol so that SLIP/PPP packets can be successfully transmitted and received across the Internet.  

Key functions of the Cisco L2F have been covered by L2TP , which is the IETF standard protocol for tunneling.


Protocol Structure - L2F: Layer 2 Forwarding Protocol



1

1

1

1

1

1

1

1

1

1

1

1

1

16

24

32bit

F

K

P

S

0

0

0

0

0

0

0

0

C

Version

Protocol

Sequence

Multiplex ID

Client ID

Length

Offset

Key

  • Version - The major version of the L2F software creating the packet.
  • Protocol - The protocol field specifies the protocol carried within the L2F packet.
  • Sequence - The sequence number is present if the S bit in the L2F header is set to 1.
  • Multiplex ID - The packet multiplex ID identifies a particular connection within a tunnel.
  • Client ID - The client ID (CLID) assists endpoints in demultiplexing tunnels.
  • Length - The length is the size in octets of the entire packet, including the header, all the fields and the payload.
  • Offset - This field specifies the number of bytes past the L2F header at which the payload data is expected to start. This field is present if the F bit in the L2F header is set to 1.
  • Key - The key field is present if the K bit is set in the L2F header. This is part of the authentication process.
  • Checksum - The checksum of the packet. The checksum field is present if the C bit in the L2F header is set to 1.


Related Protocols
PPP , L2TP , GRE , PPTP , SLIP

Sponsor Source

L2F is defined by Cisco.



Reference

http://www.javvin.com/protocol/rfc2341.pdf : Cisco Layer Two Forwarding (Protocol) "L2F".