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 From:  Michael Brown <knightmb at knightmb dot dyndns dot org>
 To:  m0n0wall at lists dot m0n0 dot ch
 Subject:  Re: [m0n0wall] Trouble with ADSL+2
 Date:  Wed, 21 Jun 2006 14:11:02 -0500
I think his ISP should be able to provide him with their MTU settings.  
That way he won't have to guess and test to find the best one.  Another 
way to know when it's broken is to ping with the no fragment flag set 
and packet sizes set to 1400 and above.  If the ping breaks after a 
certain MTU then that's as high as the packet size can get before it's 
chopped up.  In this case, probably faster just to set the MTU and ping 
until you find the largest number that will work instead of waiting for 
the ISP to give a response unless it's easy to get their tech support on 
the phone.

Thanks,
Michael

>
> Yes.  1400 is just a good number to test with.  Using the highest
> possible MTU with things still working will give you very slightly
> better performance (slightly fewer packets on large transfers).  I'd
> try 1460 next, if that works, try going up by 10 until it breaks, then
> back down until you find the sweet spot.  I'm not familiar with ADSL2,
> nor German Internet services, so don't know what the ideal MTU would
> or might be.
>
> -Chris
>
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