L2TP (Layer 2 Tunneling Protocol)
L2TP (Layer 2 Tunneling Protocol) is a tunneling protocol used for VPN. It uses the UDP port 1701 to communicate. It doesn’t have any encryption, but we can encrypt the L2TP packets by using it with IPSec. For theoretical information on L2TP you can visit its Wiki.
In Linux, you can find so many applications to build a L2TP server but there is no proper documentation for this. Also, I think no one is using L2TP VPN today. But my need could solve by L2TP only. I wanted to configure the VPN for some old devices that does support only L2TP VPN.
I use CentOS 5 Linux for VPN server. For VPN application, I use the combination to two applications. One is xl2tpd & rp-l2tpd.