OpenVPN Errors – MTU error (Maximum Transmission Unit)
Using a VPN means loading packets with a set weight size.. this is similiar to the amount of luggage you can carry on board an airplane. You must weigh your suitcases, to ensure they’re not overloaded.
Step 1 – MTU Ping Test
Start > All Programs > Accessories > Command Prompt (CMD)
ping http://www.expedient.net -f -l 1492
It will probably fail.
This means we need lighter suitcases. 🙂
Step 2 – Reduce the MTU by 10 each time.
Lower the size the packet in increments of +/-10 (e.g. 1472, 1462, 1440, 1400) until you have a packet size that does not fragment.
Step 3 – How to work out MTU
1. Use the MTU that does not fragment (ie 1472)
2. Add 28 to it for headers. (1472 + 28 = 1500)
3. Your MTU = 1500
Yay!! It’s that easy.
What would I see if I had MTU problems?
MTU problems cause severe lags in network logon times, cause email attachments and other functions within Outlook to fail, and cause applications to stop entirely.
For Windows XP:
XP has built-in PPPoE client with a default MTU of 1480.
To change the MTU, please refer to the Microsoft Knowledgebase: http://support.microsoft.com/default.aspx?scid=kb;en-us;283165 and http://www.microsoft.com/windowsxp/pro/using/howto/networking/pppoe.asp
For Windows 7:
The default MTU for Windows 7 is 1500 (Ethernet) and it autotunes the MTU, so you don’t need to reset it.
Check your MTU settings on each adapter
netsh interface ipv4 show subinterfaces
But lets say you had a burning need to adjust your Windows 7 MTU (for network reasons)…
To change the MTU for an adapter:
netsh interface ipv4 set subinterface “Local Area Connection” mtu=1458 store=persistent
netsh interface ipv4 set subinterface “Wireless Network Connection”mtu=1458 store=persistent
Hope that’s helpful for you.