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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 -f -l 1492

It will probably fail.

ping text failed

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.

ping test workedThis shows that 1472 is the correct MTU to avoid fragmentation of packets.


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:;en-us;283165 and

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

mtu adaptersSo the MTU is correctly set at 1500.

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.

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