Layer 2 Forwarding Protocol - Definition from the Hotspot Shield Glossary
This is a protocol developed by Cisco for tunneling across public networks and the internet. L2F uses tunneling protocols including PPP (Point to Point Protocol) and SLIP (Serial Line Internet Protocol), and for authentication it uses RADIUS (Remote Authentication Dial In User Service) and QoS (Quality of Service).
In an example of L2F, the PPP protocol will provide a means of connection, and L2F extends the connection to allow it to pass to the intended remote end destination. Although L2F was extremely popular and widely used, in recent developments it has been greatly superseded by L2TP (Layer 2 Tunneling Protocol) which is an extension of PPTP.