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If you have any or all of the following symptoms your MTU (Maximum Transfer Unit) may be incorrect.

•Can not access certain web sites or see particular frames within a website Websites like www.cnn.com and www.weather.com are commonly referenced.
•Web pages stall and/or eventually time out and fail to load.
•Can not access web sites or sections of web sites that require a username and password (i.e. banking, stock quote, online ordering, etc). Websites like www.chase.com and www.fidelity.com are commonly referenced.
•Can not access or download from certain FTP sites.
•Can not access certain servers or information while using a VPN client (especially active directories).
•Can not access web based or POP based email or send/receive emails.
•Can not access remote desktop applications or active directories.
•Can not send/receive file transfers.

Very, Very, Very Important Recommendation: This rest of this FAQ is for pointy headed people with no life that are just dying to get their command prompt fix and do some interesting (albeit completely boring) tests. I strongly recommend you download DrTCP, set your MTU on your NIC(s) to 1492 (or less) as shown here, and do something better with your time. That said, if you wish to continue please proceed.

Quick Tips:
•One way to verify whether if it is an MTU problem is to try and access the application or website via dial up access. Since dial up uses a default MTU of 576 bytes you will not have the same problems as broadband. If you have problems with both broadband and dial up access then the problem is something else.
•This ping test only verifies whether the packet is fragmented at the end points. •Due to additional complications, VPNs require a different type of MTU test. Please refer to the VPN MTU Test.
•Please reference the MTU troubleshooting article from Cisco for a brief overview of the MTU/PPPoE problem.
•If you have a network with multiple PCs every computer should be set up with the same MTU. Additionally, some PCs may use several Network Adapters or a VPN client adapter on one PC so you must verify you are changing the Network Adapter associated with your broadband service or VPN client.
• The built in PPPoE client for Windows XP uses an MTU that can not be easily changed. The MTU is set to 1480. For more information please reference this XP MTU article. This only applies if you are running the built in XP PPPoE client!

Finding the Correct MTU
To find the correct MTU for your configuration you must run a simple DOS Ping test. You will simply send out ping requests and progressively lower your packet size until the packet no longer needs to be fragmented. Although this simple test is accurate for testing end points, users may find that a lower MTU may be better for their particular circumstances. Please reference the following steps:

The command for this ping test is ping www.dslreports.com -f -l xxxx.
•There is a single space between each command.
•"-l" is a lower case letter L, not the number one.
•The last four numbers are the test packet size.

Step 1
Open a DOS prompt screen by clicking on Start>Programs>MSDOS-PROMPT. You can also use the Run Command by clicking on Start>Run then type in "command" for Windows 95/98/ME or "cmd" for Windows 2000/XP.

Step 2
At the DOS Prompt type in ping www.dslreports.com -f -l 1472 and hit Enter. Notice that the packet needs to be fragmented. (Figure 1)



Step 3
Drop the test packet size down (10 or 12 bytes) and test again. Notice that the packet still needs to be fragmented. (Figure 2)



Step 4
Drop the test packet size down more and test again until your reach a packet size that does not fragment. (Figure 3)



Step 5
Once you have a test packet that is not fragmented increase your packet size in small increments and retest until you find the largest possible packet that doesn't fragment.

Step 6
Take the maximum packet size from the ping test and add 28. You add 28 bytes because 20 bytes are reserved for the IP header and 8 bytes must be allocated for the ICMP Echo Request header. Remember: You must add 28 to your results from the ping test!

An example:
1440 Max packet size from Ping Test
+ 28 IP and ICMP headers
1468 Your optimum MTU Setting


Step 7
You can dowload DrTCP here or find a similar registry editing application to change your MTU. An example of changing the MTU using DrTCP is shown below. (Figure 4)


Pictures by Andy Houtz


Important Additional Notes
•There may be more than one network adapter showing in the pull down menu for your PC. You must make sure you change the MTU on the correct network adapter associated with your broadband connection or VPN client. You must also reboot your computer in order for the new MTU settings to take place. Additionally, if you have a network with more than one computer, all NICs, adapters, and router(s) must have the same MTU setting. Please reference the links below to learn how to change the MTU on some popular routers:
MTU change on a Linksys Router
•Some Linksys router model/firmware have an "Auto" MTU capability that does not function correctly. You must enable the manual MTU function and adjust the size to 1492 (or less).
MTU change on a D-Link Router

Andy Houtz DSL


Feedback received on this FAQ entry:
  • on Mikrotik on PPP Profile "Change TCP MSS" to "yes" that fixes it with Motorola Canopy Radio problems for sure

    2010-10-21 10:50:47



Expand got feedback?

by Andy Houtz See Profile edited by FAQFixer See Profile
last modified: 2008-08-13 15:47:56