The L2TP connection attempt failed because the security layer (Can;t connect to VPN)

[Kenn_Val18]

Hi @All ,

Can you help me or give me some ideas on how to solve this VPN error on Windows 10: 'The L2TP connection attempt failed because the security layer encountered a processing error during initial negotiations with the remote computer'? Some users in our company cannot connect to our VPN when they are on public Wi-Fi or other internet connections, but they can connect using some other networks. We are using a Cisco Meraki MX 17.10.4.

Thank you.

[alemabrahao]

Have you checked the logs on Windows event viewer? 

Take a look at the troubleshooting doc.

https://documentation.meraki.com/MX/Client_VPN/Guided_Client_VPN_Troubleshooting

[Kenn_Val18]

I'll check this, thank you.

[Brash]

The error message itself is not super helpful at pointing a cause.

However from what you've described where it commonly can't connect from public wifi, it could be that outgoing VPN ports are blocked.

A packet capture on each side would verify this.

[Kenn_Val18]

Do you mean that you need to open the port of the router or modem on the user's side so it could not be blocked? 

[Brash]

Yes, but I'm more talking about the internet connection that the user is on.

Public connections and even some ISP's will often block UDP 500 and 4500 used for L2TP.

[alemabrahao]

Yes, it can be, but I find it interesting that you get the error code in the Windows event viewer.

I've seen cases that indicated port blocking and in the end, it was resolved by updating the system.

Have you thought about using Anuconnect?

[CMIT-Dub]

We are receiving the same error The L2TP connection attempt failed because the security layer encountered a processing error during initial negotiations with the remote computer with Error 789 in Windows application log. We seem to have exhausted every resolution provided searching the web along with those here. What is odd is that we have one computer (Acer) Win10 that is current and successfully connects, however no other computer (HP, Dell) Win10 or Win11 will connect and are receiving this error. 

[alemabrahao]

Have you thought about using Anyconnect instead of an L2TP connection?

[CMIT-Dub]

I had not, so I enabled and configured Anyconnect and that works for us. Thanks!