Feb 192008
 

Maximum Transmission Unit (MTU) in simple words is the maximum IP packet size in bytes, that can be transmitted over the underlying network. While most of time this never has to be changed from the default values there are instances when you need to alter the MTU sizes to fix certain network performance issues. A best and classic example would be with a Windows Exchange Server being accessed over an Internet VPN setup. This in most of the cases demonstrate the problem with MTUs and related issues.

In Windows, changing the MTU value from the defaults, require you to modify the TCP/IP parameters for the adapter in Windows Registry.

WARNING: Editing Windows Registry incorrectly can cause serious problems that may require you to reinstall your operating system. I do not guarantee that you can solve problems that result from editing registry. Use Registry Editor at your own risk.

1. Click Start – RUN and type “regedit” and press enter.

2. Navigate to the following Registry Key

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters\Interfaces\<AdapterID>

HKEY_LOCAL_MACHINE
\SYSTEM
\CurrentControlSet
\Services
\Tcpip
\Parameters
\Interfaces
\<AdapterID>

In the right-panel, create a new DWORD named “MTU” with the value (say 1350 in decimal)

NOTE: To create a new DWORD, right-click, select New and select “DWORD Value” and enter the name (MTU here)

Create DWORD Value

MTU Registry DWORD

This sets the Maximum Transmission Unit (MTU) for that particular network adapter.

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  20 Responses to “How to manually set the MTU size in Windows XP 2003 2000 Vista”

  1. So how do you find out the Adapter ID? I have 10 different adapters on my PC. This is NOT helpful at all.

  2. To identify the interface, the following can help:
    - Connect if not already the case with this interface.
    - verify the interface has an IP (cmd prompt -> ipconfig /all).
    - check other registry keys in the various (10 for you) interfaces for an entry like IPAddress or DhcpIPAddress to find the IP of your interface (refresh the registry if you connected the interface after opening the registry).
    - Once found, you got it.

  3. Hi This Good Information I have sloved my problem with this information good very good keep it up

  4. Mi?a if you are not able to indentify your adapter maybe you don’t need to change MTU… Because you have not idea of what you are doing

  5. MS sez this:

    http://support.microsoft.com/kb/314053

    I’ve been grasping at straws trying to solve an odd problem that seems to have started in the last 6 months or so, noticed after I added a second home computer. Have roadrunner, surfboard cable modem, linksys befsr41 V2 firmware 1.47. Only when both computer are on, the packet loss is horrible – 20-30 % to all sites. With only one one, or when plugged directly into the surfboard with either computer, only get occasional single lost packet.

    I run a large network but this simple problem is baffling me. I don’t have another Linksys so I can’t easily do divide and conquer.

    Raargh

  6. Thanks a lot . knowledge at Top of this page is great and i had same problem and i applied this knowledge and guess what? it worked. Thanks again..

  7. This is ridiculous in linux this is done with “ifconfig adapter mtu ####”. Having to do something like this in the registry is absurd. And the fact that the adapter id is a GUID is even more absurd.

  8. It worked! thanks.

  9. It might seem easy in Linux, but then again you’ll have to figure out what your adapter name is anyways – it might be en0, wifi0, en2 etc etc. It’s not something that’s easy in either OS :P

  10. ifconfig

    That’ll show all network interfaces on linux and their names. Agreed that this registry nonsense shouldn’t be necessary. There’s an “advanced” button on the adapter properties window for a reason…

  11. @linux fanboys

    Who cares how you do it in Linux? This is a WINDOWS support site (re: windowsreference.com). When you design your own O/S or hell, even make Linux something more usable than a dog turd on the floor, maybe people will care what you think. This was useful to several of us that use a functional desktop OS – try it sometime.

  12. We are having a problem with copying files and getting the error path is too deep.

    This only happens with one particular file.

    An executable, however when we compress it into a zip it works.

    Will try setting MTU to see if this helps.

    Thanks.

  13. DDown boys!!! Heel!!! Many of us use mutltiple systems. A little extra information is quite helpful. I am glad that you like the one (and it looks like “only”) operating system you might be familiar with. No sense knocking good information because you might, for some strange reason, feel that it threatens you somehow. Many people find the concept of “computing” to be much more interesting than knowing one trick on one OS.

  14. The instructions do not indicate if reboot is needed after changing the registry or if something less intrusive is available such as disable/enable the network adapter. The instructions would be more complete with this info.

  15. Vista has no mtu listed in registry

  16. No need to reboot, just disable adapter and re-enable it and MTU is set. First I did a test with
    ping 172.19.3.1 -f -l 1273 it says:
    Packet needs to be fragmented but DF set.
    Note length of 1272 is ok (this is ping packet size, add 28 for MTU).
    Then did the registry hack increasing MTU from 1300 to 1500, disable/re-enable adapter and all is fine now up to packets of length 1472.

  17. I chased this issue on a 32 bit Windows 7 box all morning. I noticed that is was just the root dir of the mapped drive that was crashing explorer. I used a different Windows XP box to delete most all of the files from the root directory so that all I had left was the new file I really wanted.

    Bingo, all is good. I assume that there was a corrupt file in the root directory that was driving Windows 7 nuts.

  18. This worked great. Thank you so much! I don’t know why Microsoft doesn’t have this simple solution listed as KB article.

  19. This info was very helpful, stabilized my dsl speed. Before it would randomly be at max speed & drop to 1/4 of what I am suppose to be running at. Thanks for the tip!

  20. MTU has nothing to do with packet size. MTU is regarding FRAMES, not packets…just FYI

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