Therefore, if the virtual private network (VPN) server is behind a NAT device, a Windows Vista-based VPN client computer or a Windows Server 2008-based VPN client computer cannot make a Layer Two Tunneling Protocol (L2TP)/IPsec connection to the VPN server.

May 23, 2017 Setting up a L2TP over IPsec tunnel between RUT and Mar 19, 2020 How to permit l2tp ipsec vpn through Mikrotik firewall Aug 08, 2019

[SOLVED] Setting up L2TP/IPSec behind SonicWALL

Mar 19, 2020 How to permit l2tp ipsec vpn through Mikrotik firewall

What Is L2TP (Layer 2 Tunneling Protocol)? | CactusVPN

linux - Which ports for IPSEC/LT2P? - Server Fault Ipsec needs UDP port 500 + ip protocol 50 and 51 - but you can use NAt-T instead, which needs UDP port 4500. On the other hand L2TP uses udp port 1701. If you trying to pass ipsec traffic through a "regular" Wi-Fi router and there is no such option as IPSec pass-through, I recommend opening port 500 and 4500. Configuring Layer 2 Tunneling Protocol (L2TP) over IPSec