MTU, fragmentation and Jumbo Frames question

Ilya Zhuravlev i.a.zhuravlev at
Tue Feb 9 12:55:24 UTC 2010

On 09.02.2010 20:12, Harald Schmalzbauer wrote:
> Hello all,
> I have 3 machines connected to an JumboFrame enabled switch.
> One FreeBSD 8-stable and two windows machines.
> I can send echo requests with payload of 8972 between the windows
> machines, but I don't get an answer from the FreeBSD machine. At the
> edge of 8130 bytes, the FreeBSD machine eats the packets without any
> error notification. A payload <= 8130 bytes works!
> All MTUs are set to 9000, interfaces are all intel em.
> Like mentioned, the ping between the two windows machines work like
> expected, beyond 8972 bytes payload the OS is fragmenting (resp. tells
> me that DF bit was set but fragmentation was needed)
> FreeBSD seems to never fragment packets, since I don't get an answer if
> I define payload greater than MTU. But this should work, shouldn't it
> (`ping -s 10000 host`)
> Does anybody have any explanation why pings are working up to 8130 bytes
> payload and are silently droped beyond that even my MTU ist set to 9000
> (and route get confirms the MTU 9000)?

but "ifconfig em0 -rxcsum" is enough for me.

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