Layer 2 Forwarding Protocol

Short description: Cisco VPN tunneling 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]

## Contents

* 1 Use
* 2 L2F packet structure
* 3 Other VPN protocols
* 4 References
* 5 External links

## 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

## References

1. ↑ RFC2341 on IETF.org

## External links

* L2F on Cisco.com
* VPDN on Cisco.com
* L2TP on Cisco.com
* RFC2341 on IETF.org
* RFC2341 on RFC Archive

* v
* t
* e

Virtual private networking

Communications protocol|

* SSTP
* IPsec
* L2TP
* L2TPv3
* PPTP
* Split tunneling
* SSL/TLS
* (Opportunistic: tcpcrypt)

Free software|

* Cloudvpn
* FreeLAN
* FreeS/WAN
* Libreswan
* n2n
* OpenConnect
* OpenIKED
* Openswan
* OpenVPN
* Social VPN
* SoftEther VPN
* strongSwan
* tcpcrypt
* tinc
* VTun
* WireGuard
* Shadowsocks

Vendor-driven protocols|

* Layer 2 Forwarding Protocol
* DirectAccess

Proprietary software|

* Avast SecureLine VPN
* Check Point VPN-1
* Cisco Systems VPN Client
* LogMeIn Hamachi
* Microsoft Forefront Unified Access Gateway
* Hola
* Tunnelbear
* NordVPN
* SaferVPN
* ExpressVPN
* ProtonVPN
* PureVPN
* VPN.ht
* Private Internet Access

Risk vectors|

* Content-control software
* Deep content inspection
* Deep packet inspection
* IP address blocking
* Network enumeration
* Stateful firewall
* TCP reset attack
* VPN blocking

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