On Mon, 19 May 2014 23:49:22 +0930, David Newall said: > How does a packet get fragmented in this case? Does it only happen when > bridging to a device with smaller MTU? That scenario sounds quite > un-bridge-like. It also sounds like something that can be handled by > real routing. Which doesn't change the fact that you *will* get clowns who take a box that has a 10G card on a jumbogram-enabled subnet that's running with an MTU of 9000, and a 1G at MTU 1500 on the other, and try to bridge rather than route. (Did you know that you can actually mount an NFS filesystem across that? And that ls and cat and friends will work *just fine*? Until you hit a file that's more than 1.5 in size, that is. And when you do a traceroute to the wedged client, it tells you it's on the 10G network, so you have no idea why you're seeing an MTU issue. Don't ask how I know this - let's just say that supporting HPC users is never boring. :) So yes, we *do* need to do something sensible there - either frag the packet on the way out, or something. It *would* be nice if we could drop the packet and send an ICMP Frag Needed back - except it's unclear what IP you use as the source address for the ICMP....
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