MTU / Router-Access

Discussion in 'Cisco/Linksys Wireless Routers' started by coordes, Jul 27, 2005.

  1. coordes

    coordes Network Guru Member

    Hi all,
    i have just changed the MTU of the router from default (1500) to 1492.
    now i can´t connect to it via the remote access (i get an server not found).
    please help!!!
    thanks in advance,
  2. oopsibrokeit

    oopsibrokeit Network Guru Member

    when you say remote access, do you mean in your local network, just typing the IP into your browser window, or do really mean remote access, ie trying to access the unit from out in the big wide world via the internet?

    if you really are remote, then changing the mtu has probably stopped your inbound access to it, if you are nor really remote, it's probably just had a bad reaction to you changing it. rebooting it by pulling the power may well bring it back.

    If you are really really stuck you will probably have to reset it with the button and then reconfigure it from scratch.
  3. coordes

    coordes Network Guru Member

    yes, i was really stucked!

    i meant a real remote access through ddns over the internet!
    i solve the problem by getting direct access to the router within the intranet and changing the mtu back to 1500 (this works without reseting)!

    i thought there was another possibility e.g. by changing the mtu of my client-pc?!

    this experience makes me a little bit smater for such experiments the next time :wink:
  4. DocLarge

    DocLarge Super Moderator Staff Member Member,39044847,39089320,00.htm (Tweaking Win2k registry to improve tcp/ip performance)

    This has made alot of difference in better understanding MTU setting for me.

    In the future, the first thing you can try is setting the MTU on your router to "auto" and 1492 first. If that doesn't work, then try setting it to "manual" and try 1492 or 1458. If you're still getting no luck, then here's a definite way to know what the exact size packet setting you need is.

    Open a command prompt and type the following:

    ping -f -l [packet] [WAN gateway]

    Were you see "packet" is where you'd type the size of the packet you want to use and "WAN gateway" is the gateway ip provided by your ISP. So, an example would be:

    ping -f -l 1450

    If your ping comes back saying "Packet needs to be fragmented but DF set" the MTU you're choosing is still too high. Just keep whittling it down in increments of 10 or 20 and you'll soon find the right size. When you finally get a normal ping return, this means you've found an acceptable size.

    Once you find the right MTU setting that fits your needs, you can use DR TCP in a more exact manner instead of having to guess. If you read the article and see the levels you have to go to change it manually (which I've done for shitz) you can see why somebody creatd Dr TCP. If you do go the "regedit" route, the MTU will be in hexadecimal. The best way to figure out the MTU hexadecimal value is to open calculator (calc.exe from the "run" command), set the view for scientific; choose hexadeciamal and type in the hexadecimal value you see then change the calculator radio button from "hexadecimal" to "decimal;" there's your MTU!

    FYI, default MTU for 2000/XP is normally 1450-1458 and sometimes has a little bit to do with your ISP's WAN technology.

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