--_000_DB6P190MB02801B5C2E03D2E5CA6AB7B3C88A0DB6P190MB0280EURP_ Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable Exactly Fabrice! In this case the router will fragment the "bigger" MTU to = fit the "smaller" MTU but only when the DF is not set. However, fragmentati= on on routers are made by the control plane, meaning you will overload the = router CPU doing too much fragmentation. On a good NIC the announced MTU to= the IP stack is very big (like 64Kb) because the off-load engine will frag= ment this very large MTU and send it. But on this kind of NIC the fragmenta= tion is done by dedicated AISCs that does not require any CPU intervention = to do it. Just give it a try... Assemble a lab using Linux and you will see= what I am trying to explain. Moacir ________________________________ From: Fabrice Bacchella <fabrice.bacchella@orange.fr> Sent: Tuesday, August 8, 2017 2:50 PM To: Moacir Ferreira Cc: users@ovirt.org Subject: Re: [ovirt-users] Users Digest, Vol 71, Issue 37 Le 8 ao=FBt 2017 =E0 14:53, Moacir Ferreira <moacirferreira@hotmail.com<mai= lto:moacirferreira@hotmail.com>> a =E9crit : But if you receive a 9000 MTU frame on an "input" interface that results se= nding it out on an interface of a 1500 MTU, then if you set DF bit the fram= e will just be dropped by the router. The frame will be dropped and the router will send an ICMP message "packet = to big" to the sender, it's network stack will received that, learn that th= e PMTU is lower and try with smaller fragment, see https://en.wikipedia.org= /wiki/Path_MTU_Discovery. --_000_DB6P190MB02801B5C2E03D2E5CA6AB7B3C88A0DB6P190MB0280EURP_ Content-Type: text/html; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable <html> <head> <meta http-equiv=3D"Content-Type" content=3D"text/html; charset=3Diso-8859-= 1"> <style type=3D"text/css" style=3D"display:none;"><!-- P {margin-top:0;margi= n-bottom:0;} --></style> </head> <body dir=3D"ltr"> <div id=3D"divtagdefaultwrapper" style=3D"font-size:12pt;color:#000000;font= -family:Calibri,Helvetica,sans-serif;" dir=3D"ltr"> <p>Exactly Fabrice! In this case the router will fragment the "bigger&= quot; MTU to fit the "smaller" MTU but only when the DF is not se= t. However, fragmentation on routers are made by the control plane, meaning= you will overload the router CPU doing too much fragmentation. On a good NIC the announced MTU to the IP stack is very big (like 64Kb) be= cause the off-load engine will fragment this very large MTU and send it. Bu= t on this kind of NIC the fragmentation is done by dedicated AISCs that doe= s not require any CPU intervention to do it. Just give it a try... Assemble a lab using Linux and you will se= e what I am trying to explain.<br> </p> <br> Moacir<br> <br> <div style=3D"color: rgb(49, 55, 57);"> <hr tabindex=3D"-1" style=3D"display:inline-block; width:98%"> <div id=3D"divRplyFwdMsg" dir=3D"ltr"><font style=3D"font-size:11pt" face= =3D"Calibri, sans-serif" color=3D"#000000"><b>From:</b> Fabrice Bacchella &= lt;fabrice.bacchella@orange.fr><br> <b>Sent:</b> Tuesday, August 8, 2017 2:50 PM<br> <b>To:</b> Moacir Ferreira<br> <b>Cc:</b> users@ovirt.org<br> <b>Subject:</b> Re: [ovirt-users] Users Digest, Vol 71, Issue 37</font> <div> </div> </div> <div><br class=3D""> <div> <blockquote type=3D"cite" class=3D""> <div class=3D"">Le 8 ao=FBt 2017 =E0 14:53, Moacir Ferreira <<a href=3D"= mailto:moacirferreira@hotmail.com" class=3D"">moacirferreira@hotmail.com</a=
> a =E9crit :</div> <br class=3D"Apple-interchange-newline"> <div class=3D""> <div id=3D"divtagdefaultwrapper" dir=3D"ltr" class=3D"" style=3D"font-style= :normal; font-weight:normal; letter-spacing:normal; orphans:auto; text-alig= n:start; text-indent:0px; text-transform:none; white-space:normal; widows:a= uto; word-spacing:0px; font-size:12pt; font-family:Calibri,Helvetica,sans-s= erif"> <div class=3D"" style=3D"margin-top:0px; margin-bottom:0px">But if you rece= ive 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 fram= e will just be dropped by the router.</div> </div> </div> </blockquote> <br class=3D""> </div> The frame will be dropped and the router will send an ICMP message "pa= cket to big" to the sender, it's network stack will received that, lea= rn that the PMTU is lower and try with smaller fragment, see <a href= =3D"https://en.wikipedia.org/wiki/Path_MTU_Discovery" class=3D"">https://en= .wikipedia.org/wiki/Path_MTU_Discovery</a>. <div class=3D""><br class=3D""> </div> </div> </div> </div> </body> </html>
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