Question : Problem: L2TP protocol

What is L2TP protocol ?
When do we use , and for what ?
Is it the CISCO proprietary ?
Is it the Protocol for VPN session ??

thanks in advances.

Answer : Problem: L2TP protocol

L2TP is a tunneling protocol.
It is not Cisco proprietary.
It is a protocol used in one type of the VPNs; layer 2 VPNs.

Let me explain:
When you connect to the internet and connect to a host in the internet, you go through several ISPs so several routers, switches etc. Another way to connect to that machine is a through a modem connection. Your modem calls a modem connected to your target machine.

Tunneling in the internet emulates this modem connection. Although you go through several intermediate machines in order to reach your target machine in the inernet, if you use a layer 2 tunneling protocol like L2TP, you feel like you are directly connected to your target.

L2TP creates a new packet which includes your original packet(so for example if you are part of a network in which private IPs are used, you are connceted to the target with this 'non-routable in the internet' IP address.). As this is a layer 2 tuneling protocol, you are like in that target LAN.

regards...