MTU Troubleshooting

What Is MTU?

What Is Impacted By MTU?

Actually, everything can be impacted by it, but if your connection is working and only some things are problematic, like those mentioned below, then MTU is one possible cause.

There are a few services that can be dramatically impacted by MTU size. The first is a VPN (Virtual Private Network). The reason for this, is that you've created an encrypted network within a network. As such, your VPN connection has some overhead that uses up part of those 1492 or 1500 bytes of data. Having improper MTU settings can fragment those packets and cause a number of problems.

If you work for a government or a company which allows you access to your office network even when you're away from the office, then you are likely using a VPN. Usually, you have to run a special program or go to a website and sign in before you can gain that access.

Unfortunately, IT departments often don't even understand their own VPN software well enough and run into frequent problems with it, especially when setting up a new connection for an employee, with an ISP they haven't yet encountered.

Slow speeds, miscellaneous websites not working, intermittent connections only when using your VPN and even some e-mail issues are examples of problems you can experience. Keep in mind that no ISP bares any responsibility for a VPN connection, unless they actually provided it, and few if any, do. So, the responsibility for supporting VPN's falls squarely in the laps of those who set them up. In other words, it's a non-supported issue for your ISP. However, lots of people use them, so they are a reality of today's Internet. Therefore, if your Internet connection works great, except when using your VPN, then the software may be misconfigured and MTU is just about the only setting that you can personally have any control over, which may help minimize or eliminate the issues.

How Do I Diagnose An MTU Problem & What Do I Do Next?

Did you read all of that? I apologize if you did. It's pretty dry material, but maybe you gleaned some knowledge from it. Now, let's get down to business and actually do some more troubleshooting.

If you're using a router, then MTU will be covered in Router troubleshooting. I'm not going to cover that again here. Otherwise, let's continue.

Find Out Your MTU The Old School Way

The Easy Method

Warning: Grab a pen and paper to write down your settings before you change anything, in case you need to undo it at some point. Whenever you actually apply changes in this program it asks you if you want to make a backup, which of course, you do. You can restore these at a later date by utilizing the File menu. You can also Import and Export settings, if you so choose. However, it's always a good idea to keep a written copy of it, just in case the backup gets lost or damaged.

For this next part you're going to need to grab a copy of this program: TCP Optimizer.

This program doesn't need to install. It's a single file executable, so you can just click it and select Run or Open in your browser to get started. Once it's running it should be fairly self-explanatory:


Check Your Current Settings:

  • At the bottom of the window, make sure that the dot for Choose settings is set to Current.
  • Now, look at the MTU setting and write that down.
  • If things ever get really messed up, you can always select the option for Windows Default and Apply those in order to make your connection usable again.

If You Have A VPN:

  • VPN software usually installs it's own Network Adapter. It's a software component that mimics a physical network card. You will likely need to record the current settings for that by selecting it from the drop-down box, as well.
  • You can also adjust it separately. The name should be fairly obvious. It will likely include VPN or the name of the actual software you use to connect in it's description like this: VPN Tunneling Adapter.

Adjust Your Settings:

  • Slide the Connection Speed ruler at the top of the window across the bar to the value closest to your advertised connection speed. You will see the results in the line above the slider, as you move it.
  • Now, look at the Network Adapter selection. The correct Ethernet controller should already be in the drop-down box. If not, you might have to select it.

Next, do only one of the following:


  • If you're on DSL, which is high speed Internet through your phone lines, or use any ISP that requires a PPPoE connection, then move the dot at the bottom to Custom, place a checkmark in the PPPoE box in the middle of the window, on the right side, below MTU. Now move the dot at the bottom to Optimal and press Apply changes. Notice that the MTU value is set to 1492, which is the default value for PPPoE and you never likely want to adjust that to anything else.

All Other Connections:

  • Move the dot at the bottom from Current to Optimal and make note of what the program recommends you use for MTU. Write this down as well, in case you need to undo the changes at some point and press Apply changes.

By default the program will make a backup. Do yourself a favour and don't disable that. It will show you the changes you're making and ask you before continuing. Once it's done it will also ask to reboot your system, for the new settings to take affect. Let it do so.

You're done.

By this point in the process you've either determined your MTU and optimized it with TCP Optimizer, or you've determined that your current MTU results are suspicious. If you were able to optimize your MTU, hopefully the issues that you were seeing prior to this exercise have now disappeared. If not, return to the previous page and continue on with your troubleshooting.

If your MTU is suspect, you've recorded whatever results you do have in order to provide them to your ISP.

If you were troubleshooting a VPN connection, then you've recorded the results both when using the VPN software and when not using it, so that you can provide these to your IT department.

Created: 19 Jul 2011 17:04


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