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<div class="moz-cite-prefix">Thanks!<br>
<br>
But those examples are for same MTU for client and server. If we
have<br>
<br>
Client: 1500<br>
router in the middle 1500<br>
OVS/GRE: 1458<br>
server: 1458<br>
<br>
For tcp this is ok. But can it hurt somehow other protocols? UDP,
RST, etc?<br>
<br>
On 03/14/2015 08:52 PM, Joseph Bajin wrote:<br>
</div>
<blockquote
cite="mid:CAGm6i3k84rwSEMw1_ReFz=dWzVhY0mxGWAPU2h7LMsKZxX_B2Q@mail.gmail.com"
type="cite">
<div dir="ltr">The size of MTU only really matters for the server
and client. The between connections need to be larger than the
packets that are being sent.
<div><br>
</div>
<div>Scenario 1:</div>
<div>Server - 1400 MTU</div>
<div>Client - 1400 MTU </div>
<div>Switches - 9216 MTU</div>
<div>OVS - 1500 MTU</div>
<div><br>
</div>
<div>Result: Successful - Traffic passes without any issue</div>
<div><br>
</div>
<div>
<div>Scenario 2:</div>
<div>Server - 1520 MTU</div>
<div>Client - 1520 MTU </div>
<div>Switches - 1516 MTU</div>
<div>OVS - 1500 MTU</div>
<div><br>
</div>
<div>Result: Failure - Traffic will have issues passing
through. </div>
</div>
<div><br>
</div>
<div>So just make sure everything in-between is higher than your
server and client. </div>
<div><br>
</div>
<div>--Joe</div>
<div><br>
</div>
<div><br>
</div>
</div>
<div class="gmail_extra"><br>
<div class="gmail_quote">On Fri, Mar 13, 2015 at 9:28 AM, George
Shuklin <span dir="ltr"><<a moz-do-not-send="true"
href="mailto:george.shuklin@gmail.com" target="_blank">george.shuklin@gmail.com</a>></span>
wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0
.8ex;border-left:1px #ccc solid;padding-left:1ex">Hello.<br>
<br>
We've hit badly changes in behaviour of OVS when we switched
from 3.08 to 3.13 kernel. When runs on 3.11 or above, OVS
starts to use kernel GRE services. And they copy DNF (do not
fragment) flag from encapsulated packet to GRE packet. And
this mess up all things, because ICMP messages about dropped
GRE never reach neither source nor destination of underlying
TCP.<br>
<br>
We've fixed problems with MTU by using option for DHCP for
dnsmasq. This lower MTU inside instances. But there are
routers (router namespaces) and they are still using 1500
bytes MTU.<br>
<br>
I feel like this can cause problems with some types of
traffic, when client (outside of openstack) sending DNF
packets to instance (via floating) and that packet is
silently dropped.<br>
<br>
1) Is those concerns have any real life implication? TCP
should take in account MTU on server and works smoothly, but
other protocols?<br>
2) Is there any way to lower MTU inside router namespace?<br>
<br>
Thanks.<br>
<br>
P.S. Jumbo frames is not an option due reasons outside of
our reach.<br>
<br>
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</blockquote>
</div>
<br>
</div>
</blockquote>
<br>
<br>
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