The border router will do like any other router on the world. If the DF bit is 
set (common case) or if it's IPv6, it will not fragment but send an ICMP.

> Le 8 août 2017 à 13:34, Moacir Ferreira <> a écrit :
> 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. 
> Moacir
> From: Fabrice Bacchella < 
> <>>
> Sent: Tuesday, August 8, 2017 12:23 PM
> To: Moacir Ferreira
> Cc: <>
> 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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