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Ideal MTU settings for data path to SBS 2003 server

Posted on 2007-03-23
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My boss had several "internal error has occurred" messages this morning while accessing his office client via RWW.  He didn't have the "public computer" checkbox checked and since he's working from home he hasn't been leaving his system idle for 60 minutes or more where it would timeout.  I saw another question where Jeff/TechSoEasy mentioned ensuring no disparity in MTU settings between remote and local routers.  My PIX 506e firewall and Netopia DSL routers are configured for 1500.  The remote router (a LinkSys) had no value so I asked it be set to 1500.  Things are working for now so I'll keep an eye on it.  This situation, however, got me thinking that I have generally taken the MTU setting for granted, only having ensured over the years that it was "large enough."   I think for the PIX and the few LinkSys routers I myself have configured over the years the default was 1500.  Can the experts please advise (or point to an article) as to whether there is an optimum MTU setting for hardware along the remote data path to the SBS server?  Would this setting apply to IE (and anything else, FTP clients?) as well?  Thanks in advance for the illumination.
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Question by:pierc2
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by:brasslan
brasslan earned 83 total points
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I would not change the MTU for traffice that flows accross your internet connection, as a lot of the internet routers & servers are set for 1500.

If you wanted to use a larger MTU for local network traffic then you might get better performance as a larger packet size means fewer packets, and less overhead running accross your network.  But you would have to change the MTU on every network device (workstation, server, printer, router etc.) in order to make it work so that everything matches and talks well with each other.

But on your local lan, if the change was successful, I don't think that the performance increase would be noticible.  The drop in overhead would be too small to notice a real world performance boost.  If the change wasn't successful, then you would get an increased number of packets being dropped, and other communication problems.

Bottom line recommendation.
Leave it at 1500 :-)

Hope that helps a bit. (or even a byte)
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by:Olaf De Ceuster
Olaf De Ceuster earned 83 total points
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Agree with Brasslan. The default of 1500 works fine for lan. Besides XP changes it automatically.
The only time you want to drop the MTU is for internet access (Router/modem) when using DSL. Set MTU to automatic or 1492.
If you do have issues accessing the RWW you can drop MTU from client broadband only. (This seems to be an IIS quirk)
Hope that helps,
Olaf
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Jeffrey Kane - TechSoEasy earned 84 total points
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The "internal error has occured" issue with RWW has nothing to do with MTU.  (My recommendation for MTU was with regard to VPN connections anyhow... not RWW).

This error is generally caused by the Antivirus program running on the remote machine interferring with the VisualBasic operations taking place for the RWW connection.  Be sure that your boss has installed .Net 1.1 Service Pack 1 which resolves this issue.  http://support.microsoft.com/kb/821438

Jeff
TechSoEasy
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by:pierc2
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Okay, Jeff.  As usual, you got right to the heart of the matter.  Thanks.

brasslan and Olaf, even though I made an incorrect association between the error and MTU settings  since you did provide me with information on the latter I'm raising the points and splitting 3 ways.  Thanks.
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