[Beowulf] Mixing different MTU settings on the same LAN?

Carsten Aulbert carsten.aulbert at aei.mpg.de
Tue May 19 01:57:07 EDT 2009


Hi all, thanks for all the replies so far!

Patrick Geoffray wrote:
> If you use TCP, the kernel will negotiate the Max Segment Size (MSS) at
> connect time. So, a device using a larger MTU will safely communicate
> with a device with a smaller MTU, as long as there is no switch/router
> in the middle that uses an MTU smaller than at both ends.
> 

I *think* most stuff we use is TCP based, the only exception I can think
of are DNS and possibly NTP but there the packets are tiny anyway
(usually at least).

> Jumbo Frame does violate the Ethernet spec, but the vast majority of
> NICs support it and it's surprising to find some that do not in 2009.
> If you are only using TCP, you can safely mix MTU, as long as all the
> switches support the largest MTU used. If you use other protocols than
> TCP, the best solution is probably to buy NICs than support Jumbo Frames
> for those machines. Decent GigE NICs run for $25, so it may be a no
> brainer.

A tad more expensive since we want to perform PXE booting, but yes, it
seems I'll put an order out soonish. Right now there are only ~ 10
machines affected by that, so not a great deal.

Thanks all for your input

Cheers

Carsten
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