L2TP (Layer 2 Tunneling Protocol)
L2TP, originally defined in RFC 2661, was designed to provide dynamic tunneling for multiple Layer 2 circuits across packet-oriented data networks. It describes a standard method of tunneling that lets circuitlike connections across one or many Layer 3 networks appear as point-to-point or point-to-multipoint links between customer locations. The base L2TP protocol consists of a control protocol for dynamic creation, maintenance and tear-down of L2TP sessions; and data encapsulation to multiplex and demultiplex Layer 2 datastreams between IP-connected nodes.
L2TP has been focused on narrowband dial-up protocols. L2TPv3 extends L2TP by letting it run on higher-speed devices such as routers because of reduced overhead and the related decrease in processing chores. It also adds important new features such as increasing the session and tunnel ID space from 16 to 32 bits, which dramatically increases the number of tunnels from 65,000 to more than 4 billion.
From Light at the end of the L2TPv3 tunnel, Network World, 04/08/02.
Additional resources
Technology Brief: L2TP
Overview from Cisco.
Topic: VPNs
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