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cc: isp@FreeBSD.org
Subject: jumbo-frames on the network
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Hello!

Some newer machines on our FreeBSD/Linux/Windows network have 1000BaseT
network cards capable of jumbo frames up to 16K (mostly em(4)).

They are plugged into a big switch, which supports jumbo frames up to 9K
on each port.

However, when I try to crank the MTU up on this machines (up to the
switch's limitation), weird things start happening -- mostly NFS
problems. I can login to a remote host (wich has MTU of 1500), but it
will not be able to access the NFS partitions exported by a jumbo-frames
enhanced box.

Can different MTUs be mixed on the same wire -- in theory (who is to do
the fragmentation?) and in practice -- or do we need to isolate these
machines on a subnet of their own (so that they can keep fast-talking to
each other) and put something dual-homed between them and the rest of
the network?

Do the logins just work because no packet exceeds 1500 anyway?

Thanks for any advice!

	-mi