Windows: L2TP VPN Problem

Configure L2TP/IPsec server behind NAT-T device – Windows Server | Microsoft Learn

#1. Open regedit and go to:

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\PolicyAgent

#2. Right click > New > DWORD (32-bit) Value

#3. Type AssumeUDPEncapsulationContextOnSendRule as the key

#4. Set value to 2

• 0
  It’s the default value. When it’s set to 0, Windows can’t establish security associations with servers located behind NAT devices.
• 1
  When it’s set to 1, Windows can establish security associations with servers that are located behind NAT devices.
• 2
  When it’s set to 2, Windows can establish security associations when both the server and VPN client computer (Windows Vista or Windows Server 2008-based) are behind NAT devices.

#5. Reboot your computer

References:

• VpnClient Module | Microsoft Learn