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

Carsten Aulbert carsten.aulbert at aei.mpg.de
Mon May 18 03:37:18 EDT 2009


Hi,

maybe my bad feeling is just non-sense but when looking on the web for
information, it seems that there is no definitive answer I have found so
far. Our data network runs at MTU9000 but the last batch of test nodes
we bought seem to have problems with Jumbo frame (Yukon2 based NICs) or
cannot do Jumbo frames at all (nVidia).

I've run some early tests and these *seem* to suggest that the IP-layer
takes correctly care of this (e.g. tcpdump shows that the maximum header
length it 1514 bytes when the link is in use).

Since famous Wikipeda states "Limit varies by vendor. For correct
interoperation whole ethernet network must have same MTU. Usually only
seen in special purpose networks." on their page on
http://en.wikipedia.org/wiki/Maximum_transmission_unit

and others (netbsd-help page IIRC) suggests to separate these nodes from
the main network via a router I'm a bit confused who is right.

Anyone here knowing much more on networking and can give me thumbs up or
down?

Thanks

Carsten

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