North American Network Operators Group

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Re: Ethernet NAPs (was Re: Miami ...)

  • From: RJ Atkinson
  • Date: Thu Aug 23 10:37:02 2001

At 10:18 23/08/01, John Kristoff wrote:
>Certainly.  In a nutshell, it might be best to take steps to avoid
>fragmentation elsewhere in the network.  Perhaps a rule of thumb that
>should be stressed is to use jumbo frames if you know for sure the other
>end system(s) support it, otherwise default to 1500.

Or just use Path MTU Discovery.  

        For IP traffic that is traversing more than one layer-2 network, 
the variety in network technologies (e.g. ATM, SMDS, POS, Radio, 
Satellite, other) means that even ~1500 byte IP frames might not 
always work end-to-end.  For example, I know of several commercial 
IP/SATCOM systems that have an MTU of 576 bytes.  

        Many ISPs, not all, seem to try to engineer an end-to-end MTU 
of 4470 (that number chosen for historical reasons relating to FDDI).  
Some ISPs use a smaller MTU number and some use a higher MTU number.

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