I finally found the router solution: MTU!!!

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Level 1

118 points

I finally found the router solution: MTU!!!

Happy days!

I've posted a couple messages on this board over the past week regarding the fact that I could get online through my DSL modem, but as soon as I put my wireless router into the equation I could only load web pages by typing in DNS numbers. Typing in a web address the traditional way just left my browser spinning idly for minutes. I tried resetting network preferences about two dozen times, I used different DNS server numbers, reset the modem and the router countless times. I even used another router from work. Nothing worked.

Well, today I finally got hold of the RIGHT person when I called my local phone/internet company for the umpteenth time. When I described my dilemma and all the steps I'd tried he said, "You know, I'm not sure this will work, but it sounds like you may have a problem with your MTU setting on your router. Do your router setup again, and use the number 1460 for the MTU setting." I punched the number in, and VOILA I have internet again!!

I've seen this MTU setting (apparently it stands for Maximum Transmission Units or something like that) on my router setup page but never paid it any mind; I figured the system would input the number that needs to be there. Wrong! I guess if it is too high (or too low?) the communication between router and server gets messed up? I'm no tech geek, but that's the way I understand it.

Ya learn something new every day!!

Dennis

Posted on Oct 19, 2005 1:06 AM

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Answer 1

John Strung

Level 6

13,440 points

Oct 19, 2005 7:11 AM in response to Dennjon

MTU is basically the maximum packet size sent. The larger the packets, the faster the transmission rate, as long as the packets are not too large for the recipient to handle. An MTU of 1500 is standard for cable service and DSL that uses DCHP. If you DSL service uses PPPoE, the standard is 1492 and most modern routers will automatically switch the MTU from 1500 to 1492 if you switch from DHCP to PPPoE.

1460 is an odd number. Some British systems using PPPoA use lower numbers like that.

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Answer 2

OmegaOSX

Level 2

285 points

Oct 19, 2005 7:39 AM in response to Dennjon

When I set up my wireless router it was stopping me from using Software update and also from posting long replies to forums. Changing the MTU to 1482 fixed the issue for me (1492 was my automatic default).

I was banging my head for hours over this.

😉

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Answer 3

Dennjon Author

Level 1

118 points

Oct 19, 2005 10:55 AM in response to John Strung

Yes, John, 1460 does seem like a strange number. My router automatically defaults to 1492, so I'm not sure why that number doesn't work anymore. It always worked with the default until last week. Would it have something to do with corruption on the DSL line maybe?

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Answer 4

Dec 7, 2005 1:14 AM in response to Dennjon

I had a problem with not being able to do iChat sessions - after changing routers, the only thing that would work was an Airport. Which was a shame, because I was getting otherwise much better performance from my new Linksys WRT54GXv2 - snappier connections, much better throughput, much better range. I was prepared to take it back... but in desperation I called Linksys support.

"Try this. Sounds nuts, but reduce your MTU from 1492 to, say 1450. See how that goes."

With an aDSL PPPOE connection from the same supplier for the last 4 years, the MTU has always been 1492. The connection has always been wireless to the router, then router to aDSL bridge modem.

But now, with the non-Airport router, iChat wouldn't work.

So I reduced the MTU to 1450. IT WORKS! Amazing. Relieved.

(Note! No need to adjust the MTU setting on the Mac itself - at least not for me. Only the router.)

So - if you have a router with iChat not working, and you use PPPOE via ADSL, try a lower MTU than the standard 1492. It just might work.

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