MTU, fragmentation and Jumbo Frames question

Harald Schmalzbauer h.schmalzbauer at
Tue Feb 9 12:12:24 UTC 2010

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)?

Thanks for any help,


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