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

Michal Privoznik mprivozn at redhat.com
Thu Oct 22 14:57:53 UTC 2015

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

    Add support for <option> tag in network config
    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 at hollants.com>
    Signed-off-by: Laine Stump <laine at 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.


1: http://libvirt.org/git/?p=libvirt.git;a=blob;f=tests/interfaceschemadata/bridge.xml;h=ece087ea2ef5e3a85fd708ff44286acb15680a64;hb=HEAD

2: http://libvirt.org/formatnetwork.html#examples

P.S.: I must ashamedly admit that we don't have any documentation for <interface/>, sorry.

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