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How to connect a company or privacy VPN over 4G, such as Eir, GoMo and Vodafone
A few people asked me for help troubleshooting their workplace or privacy VPN over mobile broadband.  The main symtom was that the connection would establish, but had no connectivity other than being able to ping hosts.  While this mainly affect connections over Eir and GoMo, one had this issue with Vodafone 4G also. 

After being able to reproduce the issue with privacy VPN service over Eir 4G, it turns out that Eir uses a smaller MTU size than most fixed fixed line connections.  This also affects GoMo, which uses the Eir cellular network.  Generally a VPN connects with an MTU size of 1400, so the additional packet overhead works fine with the common 1492 and 1500 MTU sizes of most fixed line connections.    

To fix the issue, the VPN MTU size needs to be reduced.  I generally find 1300 works fine with Eir.

Note: If using a company laptop, these steps require Administrator access.
  1. Connect to the VPN
  2. Click Start and type: CMD
  3. Right-click "Command Prompt", click "Run as Administrator" and click 'OK'.
  4. Type the following command: netsh interface ipv4 show subinterfaces
  5. Look for the line with an MTU size of 1400 and a "Bytes Out" column greater than 0.  Use this interface name for step 6.  The following is an example:
  6. Type the command using the interface name from step 5: netsh interface ipv4 set subinterface "interface name" mtu=1300 store=persistent
  7. If it says 'OK', you can check that it saved by typing: netsh interface ipv4 show subinterfaces

If you got an error "The filename, directory name, or volume label syntax is incorrect.", check that the interface name in the quotes matches exactly as shown in step 5, including any dashes, dots, etc.  You can copy & paste the name by dragging the mouse over the interface name, press enter and right-click to paste.

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