A VPDN is a network that extends remote access dialup clients to a private network. VPDN tunnels use either Layer 2 forwarding (L2F) or Layer 2 Tunnel Protocol (L2TP).
Cisco introduced L2F in RFC 2341. It is also used to forward PPP sessions for Multichassis Multilink PPP.
L2TP, introduced in RFC 2661, combines the best of the Cisco L2F protocol and Microsoft Point-to-Point Tunneling Protocol (PPTP). Moreover, L2F supports only dial-in VPDN, while L2TP supports both dial-in and dial-out VPDN.
Both protocols use UDP port 1701 to build a tunnel through an IP network to forward link-layer frames.
For L2F, the setup for tunneling a PPP session consists of two steps:
Establish a tunnel between the NAS ...