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Re: [libvirt-users] How can I set the advertised value of MTU for virbr0 interface?

On 10/22/2015 10:57 AM, Michal Privoznik wrote:
On 15.10.2015 20:23, George Trakatelis wrote:
Hi all,
I want to set the advertised value of MTU
for virbr0 interface on a Fedora 21 host to 1454
to accommodate the installation of new VMs for testing purposes.
The aforementioned host is an OpenStack VM actually,
which explains the need for a lower than 1500 MTU.
I noticed Pieter Hollants' patch to set
dhcp-option=<number>,"<value>" for an interface,
but it seems that it has not been implemented yet.
The patch is in:

Author:     Pieter Hollants <pieter hollants com>
AuthorDate: Thu Feb 21 23:40:17 2013 +0100
Commit:     Laine Stump <laine laine org>
CommitDate: Fri Feb 22 19:45:19 2013 -0500

     Add support for <option> tag in network config

As you've found, that was reverted before it was ever in a release. After initially approving it, I realized that the free-form format of the option values was doomed to create problems in the future, and that we had to come up with a more robust syntax (essentially libvirt needed to know the datatype of each option value, rather than just accepting an opaque string and passing it on to the dhcp server).

This patch adds support for a new <option>-Tag in the <dhcp> block of
     network configs, based on a subset of the fifth proposal by Laine
     Stump in the mailing list discussion at
     Any such defined option will result in a dhcp-option=<number>,"<value>"
     statement in the generated dnsmasq configuration file.
Currently, DHCP options can be specified by number only and there is
     no whitelisting or blacklisting of option numbers, which should
     probably be added.
Signed-off-by: Pieter Hollants <pieter hollants com>
     Signed-off-by: Laine Stump <laine laine org>

But I don't think it will solve your problem. IIUC you want to modify MTU of virbr0 not just advertise altered MTU to domains.

Does this virbr0 come from the default libvirt network? If so, I am afraid there's no way how to specify MTU. What you could do, is to define <interface/> [1] and then bridged <network/> [2] that will use the interface.
Libvirt already has support for setting MTUs for host interfaces.

It really surprised me a month or two ago when someone asked about this on IRC and I found that we didn't support setting the MTU in a libvirt virtual network. Due to MTU being supported in host interface config, I had just always assumed in the back of my mind that it was there for networks as well, but never needed it and so never paid close attention.

But when it was pointed out it was missing, my first suspicion was that there must be some reason it wasn't done (since it's so simple).

And then (as usual) I went back to whatever I was working on and forgot about it :-/

It makes sense to me that the <bridge> element should have an attribute called "mtu" (this is a case where I think we shouldn't try to use exactly the same config as host interfaces, since they have a separate <mtu size='blah'/> element so that it can be used with any type of interface). So it would look something like this:

    <bridge name='virbr0' mtu='9000' stp='on' delay='0'/>

If someone feels like implementing this that would be great. If not, it's on my list and may get implemented "someday".

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