L2TP

(Layer 2 Tunneling Protocol) A protocol from the IETF that allows a PPP session to travel over multiple links and networks. L2TP is used to allow remote users access to the corporate network. PPP is used to encapsulate IP packets from the user's PC to the ISP, and L2TP extends that session across the Internet. L2TP was derived from Microsoft's Point-to-Point Tunneling Protocol (PPTP) and Cisco's Layer 2 Forwarding (L2F) technology. See PPTP and L2F.

From Access Concentrator to Network Server
The "L2TP Access Concentrator" (LAC) encapsulates PPP frames with L2TP headers and sends them over the Internet as UDP packets (or over an ATM, frame relay or X.25 network). At the other end, the "L2TP Network Server" (LNS) terminates the PPP session and hands the IP packets to the LAN. L2TP software can also be run in the user's PC.

Carriers also use L2TP to offer remote points of presence (POPs) to smaller ISPs. Users in remote locations dial into the carrier's local modem pool, and the carrier's LAC forwards L2TP traffic to the ISP's LNS.

L2TP and IPsec
L2TP does not include encryption (as does PPTP), but is often used with IPsec in order to provide virtual private network (VPN) connections from remote users to the corporate LAN. See PPP, VPN and IPsec.

PPP and L2TP Traffic

PPP encapsulates IP packets from the user's PC to the ISP. L2TP tunnels those packets over multiple links.

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L2TP - Layer Two Tunneling Protocol