How to calculate the maximum MTU?

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Category: General
Also available:  German (DE) 
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It's quite easy - if you know how to do it:
 

 

  1. Decrement with ping -c1 195.135.220.3 -s 1600 -M do parameter 1600 for 100 until there is no message 'DF set' any more.
  2. Increment the parameter by 10 until the message 'DF set' is ritten again.
  3. Decrement the parameter for 2 until there is no message 'DF set' any more.
  4. Add 28 (IP Headersize) -and that's the optimal MTU size
  5. With ipconfig eth0 mtu 1492 you can change the MTU size temporarily.
 
Oneliner to calculate the maxium MTU: 
size=1272; while ping -s $size -c1 -M do google.com >&/dev/null; do ((size+=4)); done; echo "Max MTU size: $((size-4+28))"
 
Since Version 0.5.1.1 collectNWData executes this Test and writes a warning message if the MTU is not optimal.