Layer 2 Tunneling Protocol (L2TP)

Layer Two Tunneling Protocol (L2TP) is an extension of the PPTP that is documented and defined in RFC 2661. L2TP is used to enable the operation of a VPN over the Internet. RFC 3193 defines using L2TP over a secure IPsec transport. In this approach, L2TP packets are exchanged over User Datagram Protocol (UDP) port 1701. IPsec Encapsulating Security Payload (ESP) protects UDP payload to ensure secure communication. Cisco and Microsoft agreed to merge their respective L2TP, thereby adopting the best features of two other tunneling protocols: PPTP from Microsoft and Layer 2 Forwarding (L2F) from Cisco.

The two main components that make up L2TP are the L2TP Access Concentrator (LAC), which is the device that physically terminates a call, and the L2TP Network Server (LNS), which is the device that terminates and possibly authenticates the PPP stream.