Why is L2TP VPN not working on Windows 11 with USG FLEX devices?

Zyxel_Emily

Question:
I set up a VPN configuration on a USG FLEX device. The VPN works fine on iPhone but fails on a Windows 11 Pro laptop.

Answer:
Windows Firewall on Windows 11 may block the required connections. Follow the steps below to resolve the issue:

• Enable firewall rules: Open Command Prompt as an administrator, type firewall.cpl and press Enter to open the Windows Defender Firewall. Click "Allow an app or feature through Windows Defender Firewall" and ensure that "Routing and Remote Access" and "Secure Socket Tunneling Protocol" are checked.

• Modify the registry: Open Registry Editor by typing regedit in the Run dialog (Win + R). Navigate to HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\PolicyAgent and add a new DWORD (32-bit) Value named AssumeUDPEncapsulationContextOnSendRule. Set its value data to 2 and restart the computer. This enables the client to connect to an L2TP/IPsec server behind a NAT device.