L2F

L2F, or Layer 2 Forwarding, is a tunneling protocol developed by Cisco Systems in the mid-1990s to support remote access VPNs by carrying PPP frames over an IP network. It defines a tunnel endpoint between a client and a network access server, encapsulating PPP frames within IP packets so a user can access a private network across an intervening IP network without exposing internal topology.

In operation, a client initiates a connection to the tunnel endpoint, negotiates a PPP link using standard PPP authentication methods, and PPP frames are transported through the IP tunnel to the remote network. L2F itself does not provide encryption; the security of transmissions relies on the PPP authentication process and the security of the underlying IP network. The protocol was designed to simplify provisioning of remote access by delegating link establishment and user authentication to existing PPP mechanisms.

L2F was eventually superseded by the Layer 2 Tunneling Protocol (L2TP), which was developed by the IETF by combining concepts from L2F and the earlier PPTP technology and adding features such as support for multiple tunnels and, in common deployments, encryption via IPsec (in L2TP/IPsec). As a result, L2F largely faded from use in favor of L2TP-based VPN solutions, which offered broader interoperability and stronger security options.

See also: Layer 2 Tunneling Protocol, PPTP, VPN.