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Windows 10 L2TP/IPsec Manual Setup Instructions
Bold items are things you will click or type
To add a necessary registry setting:
Press the Windows Key and R at the same time to bring up the Run box.
Type in: regedit and click OK. Click Yes if asked if you'd like to allow the app to make changes to your PC.
In the left pane, locate and click the folder: HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\PolicyAgent
Click the Edit menu and hover your cursor over New. Click DWORD Value. A new registry will appear in the right pane, named New Value #1.
Rename that file: AssumeUDPEncapsulationContextOnSendRule (this is case-sensitive and contains no spaces) and press ENTER.
Right-click AssumeUDPEncapsulationContextOnSendRule, then click Modify.
In the Value data box, type 2 and click OK.
Reboot the computer.
來源： https://www.giganews.com/support/vyprvpn/vpn-setup/windows-10/l2tp.html