VPN – L2TP Error 809

If you’re using L2TP with IPSec.

Windows doesn’t activate NAT-T by default (Windows 10 included) you need to add a registry key:

To create and configure the

AssumeUDPEncapsulationContextOnSendRule

registry value, follow these steps:

Log on to the Windows Vista client computer as a user who is a member of the Administrators group.
Right Click Start, click Run, type regedit, and then click OK. If the User Account Control dialog box is displayed on the screen and prompts you to elevate your administrator token, click Continue.
Locate and then click the following registry subkey:

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\PolicyAgent
On the Edit menu, point to New, and then click DWORD (32-bit) Value.
Type AssumeUDPEncapsulationContextOnSendRule, and then press ENTER.
Right-click AssumeUDPEncapsulationContextOnSendRule, and then click Modify.
In the Value Data box, type one of the following values:
- 0
  A value of 0 (zero) configures Windows so that it cannot establish security associations with servers that are located behind NAT devices. This is the default value.
- 1
  A value of 1 configures Windows so that it can establish security associations with servers that are located behind NAT devices.
- 2
  A value of 2 configures Windows so that it can establish security associations when both the server and the Windows Vista-based or Windows Server 2008-based VPN client computer are behind NAT devices.
Click OK, and then exit Registry Editor.
Restart the computer.

Validate that the windows service: IPSec Policy Agent is started.