True! But in some point of the network it may be necessary to make the MTU 
1500. For example, if your data need to cross the Internet. The border router 
in between your LAN and the Internet will have to fragment a large frame back 
to a normal one to send it over the Internet. This router will just "die" if 
you have a heavy load.


From: Fabrice Bacchella <>
Sent: Tuesday, August 8, 2017 12:23 PM
To: Moacir Ferreira
Subject: Re: [ovirt-users] Users Digest, Vol 71, Issue 37

Le 8 août 2017 à 11:49, Moacir Ferreira 
<<>> a écrit :

This is by far more complex. A good NIC will have an offload engine (LSO - 
Large Segment Offload) and, if so, the NIC driver will report a MTU of 64K to 
the IP stack. The IP stack will then send data to the NIC as if the MTU were 
64K and the NIC will fragment it to the size of the "declared" MTU on the 
interface so PMTUD will not be efficient in such scenario. If all this takes 
place in the server, then you get no problem. But if a standard router is 
configured to support 9K jumbo frame in one interface (i.e.: LAN connection) 
and 1500 in another (i.e.: WAN connection) then the router will be responsible 
for the fragmentation.

That's happen only if the bit don't fragment is not set, otherwise router are 
not allowed to do that and send back a "packet to big" ICMP, it's called path 
mtu discovery. To my knowledge, it's usually set, and even mandatory on IPv6.

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