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J57little's avatar
J57little
Newbie Caller
Hace 4 años

VPN issues

All devices and home network are good. Signal strength is 2 bars (weak, but functional).  Spouse needs to set up VPN to work from home, but even IT from the office could not get VPN to work via T-Mob...
ayuda
PeteO's avatar
PeteO
Network Novice
Hace 3 años

I was getting a situation with the VPN where it appeared to connect, but it was not functional.  The problem is the MTU packet size value.

For me, I found lowering the MTU value at the client fixed the problem.  The tmobile(nokia) modem seems to be set to 1450 and the cable company was set to 1500.  I don't think Tmobile will change it at their end. The built in Windows 10 VPN client defaults it to 1400, but there is some header information that moved it past the 1450 window.

 

  1. Find the interface-With the VPN “connected”, initiate the administrator command prompt and type netsh int ipv4 show subinterface.  This will give you a list of interfaces like this:  

       MTU  MediaSenseState   Bytes In  Bytes Out  Interface

    ------  ---------------  ---------  ---------  -------------

      1400                1    2073205    1650218  MY VPN

    4294967295                1          0     167019  Loopback Pseudo-Interface 1

      1500                1  384370968   39338930  Wi-Fi

      1500                5          0          0  Bluetooth Network Connection

      1500                5          0          0  Local Area Connection* 1

      1500                5          0          0  Local Area Connection* 2

      1500                1          0     311118  VirtualBox Host-Only Network

  2. Then, back down the MTU setting with netsh int ipv4 set subinterface “MY VPN” mtu=1380 store=persistent.  You may need to adjust the 1380 value lower for your system.  

  3. exit the VPN and then restart the VPN and see if it works.

  4. If you want to dig deeper, you can figure out when the MTU value craps out with ping www.yahoo.com -f -l -1500 and keep lowering the "1500" value until you see "Packet needs to be fragmented but DF set".  The last value that worked is close to what the modem is set to.  I'd subtract 70 from that value and set the client mtu to that.