Trouble with connection to work VPN

samgsapp1971 wrote:

I am using T-Mobile 5G home internet.  Everything works great, but all of the sudden when I connect my work laptop to the VPN, nothing over the VPN works.  This was working fine for a year.  Not sure if there was a firmware upgrade something that caused this.  After a ton of searching, I found the issue for me.  It was the MTU setting on my laptop.  This is very easy to fix.  It was as simple from changing a setting from 1400 to 1374.  That worked.  Google hot to check your MTU size.  It is with the "NETSH" command.  Do some ping tests forcing a MTU size until you find the best one.  Make the change. Very basic overview….

1.

Checking and changing the MTU size

Open a command prompt as admin

Check the MTU size as it is set now.

Netsh interface ipv4 show interfaces

2.  

   Before we change the MTU, we need to find the optimum size by using this command

   Ping 8.8.8.8 -f -l 1400

It is showing as fragmented.  We want to find the highest number where we do not get that. So keep trying numbers, lower and lower until one comes back normal.

For me, it was 1346.  That's our base number.  Fore technical reasons having to do with headers, etc, we need to add 28 to that number.  My number would be 1374.

3.

Change the mtu (CMD as admin)

Netsh interface ipv4 set subinterface “Ethernet 4” mtu=1374 store=persistent

(Ethernet 4 is the name of my interface, yours will probably be different, like “Wifi” )

Now check your change.

Netsh interface ipv4 show interfaces

You should now see the new MTU

 

 

Thanks very much for this! Looks like this could be working for me. 

However, following your testing procedures by pinging with different sized packets, when I'm not on VPN, I'm able to send packet sizes up to 1472 successfully (corresponding to MTU of 1500). When I'm connected to my VPN, the MTU was already set to 1390, but I was still having issues with VPN. I went ahead and just tried your MTU setting of 1374, and it seems like VPN is working so far (only tested a few minutes). Tried various settings around 1374, and anything larger fails, while smaller settings continue working. 

So for others, if the detected maximum MTU size doesn't resolve your VPN issues, try 1374, or possibly other settings that are smaller than the detected max MTU size. In my case, changing MTU size via netsh made immediate impact, so it was easy to just test different values to see what works.