But if you receive a 9000 MTU frame on an "input" interface that results sending it out on an interface of a 1500 MTU, then if you set DF bit the frame will just be dropped by the router. If you want your data to "cross" your frame over a different MTU path, then you can not set DF to 1. This is a quite simple and easy thing to demonstrate. Just create a simple virtual lab with 3 Linux doing routing and test it. So, if your goal is to communicate over paths that may have a MTU lower than 9000 you better make sure your server sends out a frame that the path can support.
Moacir ________________________________ From: Fabrice Bacchella <fabrice.bacche...@orange.fr> Sent: Tuesday, August 8, 2017 1:37 PM To: Moacir Ferreira Cc: users@ovirt.org Subject: Re: [ovirt-users] Users Digest, Vol 71, Issue 37 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 <moacirferre...@hotmail.com<mailto:moacirferre...@hotmail.com>> 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 <fabrice.bacche...@orange.fr<mailto:fabrice.bacche...@orange.fr>> Sent: Tuesday, August 8, 2017 12:23 PM To: Moacir Ferreira Cc: users@ovirt.org<mailto:users@ovirt.org> Subject: Re: [ovirt-users] Users Digest, Vol 71, Issue 37 Le 8 août 2017 à 11:49, Moacir Ferreira <moacirferre...@hotmail.com<mailto:moacirferre...@hotmail.com>> 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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