Configure a L2TP/IPsec server behind a NAT-T device
https://docs.microsoft.com/en-US/troubleshoot/windows-server/networking/configure-l2tp-ipsec-server-behind-nat-t-device Summary Important This section, method, or task contains steps that tell you how to modify the registry. However, serious problems might occur if you modify the registry incorrectly. Therefore, make sure that you follow these steps carefully. For added protection, back up the registry before you modify it. Then, you can restore the registry if a problem occurs. For more information about how to back up and restore the registry, see How to back up and restore the registry in Windows . By default, Windows Vista and Windows Server 2008 don't support Internet Protocol security (IPsec) network address translation (NAT) Traversal (NAT-T) security associations to servers that are located behind a NAT device. If the virtual private network (VPN) server is behind a NAT device, a Windows Vista or Windows Server 2008-based VPN client computer can't make a Layer...