Layer 2 Forwarding (L2F)

Definition of Layer 2 Forwarding (L2F) in The Network Encyclopedia.

What is L2F (Layer 2 Forwarding)?

A media-independent tunneling protocol developed by Cisco Systems. The Layer 2 Forwarding (L2F) protocol tunnels data-link layer frames in such protocols as Point-to-Point Protocol (PPP) or Serial Line Internet Protocol (SLIP), making it possible to create virtual private networks (VPNs) over a public network such as the Internet.

On the server side, L2F can be used with such features as user authentication through Remote Authentication Dial-In User Service (RADIUS), dynamic allocation of addresses, and quality of service (QoS). L2F is implemented in Cisco routers through Cisco’s Internetwork Operating System (IOS).

How it works

When using PPP with L2F, for example, PPP provides the connection between a dial-up client and the network access server (NAS) that receives the call. A PPP connection initiated by a client terminates at a NAS located at a PPP service provider, usually an Internet service provider (ISP). L2F allows the termination point of the connection to be extended beyond the NAS to a remote destination node, so the client’s connection appears to be directly to the remote node instead of to the NAS. The function of the NAS in L2F is simply to project or forward PPP frames from the client to the remote node. This remote node is called a home gateway in Cisco networking terminology.

NOTE

L2F has been largely superseded by the newer Layer 2 Tunneling Protocol (L2TP), an Internet Engineering Task Force (IETF) standard protocol that provides a vendor-neutral tunneling solution. L2TP is an extension of the PPP protocol that supports the best features of the Point-to-Point Tunneling Protocol (PPTP) and the L2F protocol.