Fix "Unsuccessful L2TP connection attempt" error in Windows 10

February 6, 2023

Layer 2 Tunneling Protocol (L2TP) mainly supports virtual private networks. Any VPN hides the source of the network connection by masking your original data with some other server data located in a different environment. Masking your geographic location will help you bypass any network path restrictions. In recent days, many of you have been complaining about an unsuccessful L2TP connection attempt because the security layer encountered a processing error while trying to establish a VPN connection. If you are in this category, this guide will help you solve this problem with effective troubleshooting ideas. So keep reading!

How to Fix "L2TP Connection Failed" Error in Windows 10

We collected a lot of data from several users and online reports and developed a set of causes causing the issue under discussion. You are advised to analyze the causes one by one and check which of the reasons listed below caused the problem.

You are using a duplicate version of the VPN key or certificate.
The established connection does not comply with Network Address Translation (NAT) rules.
The VPN client is missing a computer certificate, or the root computer certificate does not have "Server Authentication" as its EKU (Extended Key Usage).
Several important services, such as IPsec key modules and policy agent services, are not running.
The Point-to-Point protocol is incorrectly configured on your PC.
Microsoft CHAP v2 protocol unlocked on VPN server.
Outdated Windows operating system.
An antivirus or firewall is interfering with the network connection.

Now you know the reasons why an L2TP connection attempt failed because the security layer detected a processing error on a Windows 10 PC. Here you will learn troubleshooting ideas to help you solve the problem.

Method 1: Run the network troubleshooter

If any network connection issues are contributing to the L2TP connection attempt failing, you can use the built-in tool provided on your Windows 10 PC when needed. Follow our Windows 10 Network Connection Troubleshooter guide to learn the basic troubleshooting steps to fix all network related problems with the Windows Network Adapter troubleshooter.

You can use the steps in the guide to diagnose Wi-Fi, Ethernet, and all network adapter connections on your Windows 10 PC.

Method 2: Repair Corrupted Files

The presence of corrupted files on your computer always interferes with any of the Windows processes. You are advised to repair all corrupted files on your computer in order to resolve the issue in question. Read our guide to restoring system files in Windows 10 and follow the instructions to repair any corrupted files and components left on your computer.

After you have repaired the corrupted components of your computer, check if you can fix the failed L2TP connection attempt on Windows 10 or not.

Method 3: Run commands to start multiple services

This is a simple and effective method to help you fix a failed L2TP connection attempt on Windows 10 by executing simple command lines. Several users claim that this method has been helpful in resolving the issue under discussion. Here are some steps to implement the same.

1. Press the Windows key and type cmd. Click "Run as administrator".

2. Then paste the following commands one by one and press the Enter key after each command.

sc config IKEEXT start=demandsc config PolicyAgent start=demand

3. Wait for the commands to complete and restart the PC.

Check to see if the L2TP connection attempt is resolved due to the security layer encountering a processing error.

Method 4: Enable MS-CHAP v2

Before connecting the VPN service to your computer, you must make sure that the Microsoft CHAP v2 protocol is enabled or not. If not, you must enable it manually. In any case, if Microsoft CHAP v2 is disabled on your computer, you will encounter L2TP connection failure on Windows 10. Follow the instructions below to enable MS-CHAP v2 on your device.

Note. Make sure the VPN service is configured correctly and the VPN credentials are correct and match the VPN server. You can verify this by the fact that if you have established the connection correctly, you can see it in the list of network adapters.

1. Open the Run dialog box by pressing the Windows + R keys at the same time.

2. Now type ncpa.cpl and press the Enter key to start network connections.

3. Now right-click on the VPN connection network and select the Properties option.

4. Now go to the Security tab and click on Allow these protocols.

5. Now select Microsoft-CHAP version 2.

6. Finally, click OK to save your changes.

Now try to establish a VPN connection and check if you were unable to fix the L2TP connection attempt due to the security layer detecting a processing error on your computer.

Method 5: Enable Link Management Protocol Extensions

On any network, the Point-to-Point Protocol (PPP) is responsible for the transmission, compression, and authentication of any VPN connection. Therefore, the PPP settings on your network must be properly configured to allow any Link Control Protocol (LCP) extensions as they are an important part of any PPP network. Hence, follow the steps below to enable Link Control Protocol (LCP) extensions on your VPN connection.

1. Repeat steps 1-3 as in the previous method.

2. Now go to the Options tab and click on PPP Settings.

3. Then click Enable LCP Extensions.

4. Next, click OK.

5. Click OK again to save changes and connect to the VPN now.

Check if you are experiencing L2TP connection failure again in Windows 10.

Method 6: Restart Essential Services

Many users who encounter an L2TP connection failure due to a security layer processing error assume that restarting the IPsec Key Modules and IPsec Policy Agent Services resolved the issue. Here are some instructions on how to enable the listed services on your Windows 10 PC.

1. Press the Windows key and type Services, then click Open.

2. Then scroll down and double-click the IKE and AuthIP IPsec Key Module Service.

3. On the General tab, select Automatic from the drop-down menu for Startup Type.

4. Verify that the status of the service is "Running". If not, click the Start button.

5. Then click "Apply" and "OK" to save the changes.

6 Repeat the same steps for the IPsec Policy Agent service and save the changes.

8. Finally, try restarting the VPN service.

Method 7: Update Windows

Your Windows 10 PC rolls out updates to improvise features you're familiar with. Updating Windows is very important and in most cases very safe. If you want to fix a failed L2TP connection attempt on Windows 10, try to update your computer as mentioned in our guide, use our guide How to Download and Install the Latest Windows 10 Update.

After updating the operating system, check if the issue is resolved.

Method 8: Temporarily disable your antivirus (if applicable)

Sometimes your antivirus program can interfere with the VPN connection and contribute to the failed L2TP connection attempt on Windows 10. Some third-party antivirus programs, such as Norton and Avast, prevent any of the latest updates that fix errors on your computer. So follow our How To Temporarily Disable Your Antivirus On Windows 10 guide to temporarily disable your antivirus program on your PC.

After fixing the VPN issue, be sure to re-enable your antivirus program as a PC without a security package is always a threat.

Method 9: Disable Windows Firewall (Not Recommended)

In any case, if the VPN service is blocked by the Windows Defender Firewall, you need to temporarily disable the firewall securities on your computer in order to fix the failed L2TP connection attempt because the security layer has detected a processing error. If you don't know how to disable Windows Defender Firewall, our guide to disable Windows 10 Firewall will help you with that. You can also allow the VPN client by following our guide on Allow or block apps through Windows Firewall.

Once you can access the VPN service, make sure you enable the firewall package again, your computer is more susceptible to malware attacks without firewall protection.

Method 10: Reset TCP/IP settings

TCP (Transmission Control Protocol) is required for network connectivity because it sets the rules and standard procedures on the Internet. Follow our guide to troubleshoot network connection issues in Windows 10 to reset TCP/IP.

After completing the steps, check if you can fix the problem or not.

Method 11: Create a new registry entry

Few users have confirmed that creating a new entry in the Windows Registry Editor helped them fix the failed L2TP connection attempt issue on Windows 10. Here are some instructions to implement the same.

Note. It is recommended that you back up your registry before following the steps. If you don't know how to back up the registry, use our guide "How to back up and restore the registry in Windows" and follow the steps described.

1. Press the Windows + R keys at the same time to open the Run dialog box.

2. Now type regedit in the box and press the Enter key to launch the Registry Editor.

3.Now navigate to the following key folder path in Registry Editor.

HKEY_LOCAL_MACHINESystemCurrentControlSetServicesRasManParameters

4. Now, right click on the right pane and select "New" and then the option "DWORD Value (32-bit)" as shown in the picture.

5. Now double click on the key you created now, set the Value to 1 and close all windows.

Method 12: Create a UDPE Encapsulation Registry Key

Similarly, you need to create a UDPE encapsulation registry key in order to fix the failed L2TP connection attempt issue on Windows 10. This key will help you fix VPN connection issues that are causing network instability. So you can easily connect to a VPN server and here are the steps to do the same.

1. Press the Windows key and type Registry Editor. Click Open.

2. Now go to the following path in the registry editor.

ComputerHKEY_LOCAL_MACHINESYSTEMCurrentControlSetServicesPolicyAgent

3. Now right click on the right window and select "New" and then the option "DWORD (32-bit) Value" as shown in the picture.

4 Name the key AssumeUDPEncapsulationContextOnSendRule and double-click the generated key.

5. Then set the Value to 2 and click OK.

6. Finally, restart your computer.

Method 13: Manually Forward Ports

This method will redirect all communication packets from one port and destination address to another as it roams the network. This will improve the network performance of your computer and thus help you resolve the issue under discussion. Here are some manual port forwarding instructions.

1. Open a tab in your browser and go to the Portforward website and write down the default ports for the program you want to run.

2. Then find the IP address, username and password of your wireless router.

Note. If you don't know how to find your router's IP address, see our guide How to Find My Router's IP Address? or you can also find information in the router's manual.

3. Now launch any browser and enter your router's IP address in the address bar.

4. Now enter your router credentials and log in.

5. Then find any data using port forwarding. You can find it in advanced settings or other similar menus, depending on the model of router you are using.

6. Then enable Post Forwarding and click on the option that allows you to add specific ports.

7. Enter the default port number in the empty text boxes and click Save.

8. Finally, check if you can establish a VPN connection without any problems.

Method 14: Reinstall your VPN adapter

If there is any instability in your network adapters, you will experience an unsuccessful L2TP connection attempt because the security layer has detected a processing error. Follow our guide How to uninstall and reinstall drivers in Windows 10 to reinstall drivers on your Windows 10 PC.

After reinstalling the network drivers, check if you have fixed the problem in question or not.

Method 15: Roll back VPN adapters

When VPN drivers are rolled back, all current versions of the hardware device will be removed and previously installed drivers will be installed on your PC. Follow the instructions in our How to Roll Back Drivers in Windows 10 guide to fix the failed L2TP connection attempt issue in Windows 10.

Once your Windows 10 PC has the previous version of the drivers installed, check if the VPN connection issue is resolved.

Method 16: Remove the latest security updates

If you encounter a Windows 10 error when trying to connect L2TP after a Windows OS update, your VPN connection may not be compatible with the new update, in which case uninstall the latest updates on your PC. It also improves the stability of the PC after uninstalling a faulty update. Here are some instructions on how to uninstall the latest updates on a Windows 10 PC.

1. Press the Windows key, type "Control Panel" and click "Open".

2. Now click "Uninstall a program" in the "Programs" menu as shown in the picture.

3. Now click "View Installed Updates" on the left pane as shown here.

4. Now select the latest update (for example, kb:5009543 for Windows 10, kb:5009566 for Windows 11) and click Uninstall as shown below.

5. Then confirm the prompt, if any, and restart your computer.

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Now you would fix the failed L2TP connection attempt because the security layer detected a processing error in Windows 10. If you are stuck in the middle of the troubleshooting process, please let us know in the comments section. Feel free to contact us for additional questions.