----- Original Message -----
> From: "Igor Lvovsky" <ilvov...@redhat.com>
> To: "VDSM Project Development" <vdsm-devel@lists.fedorahosted.org>
> Cc: "Simon Grinberg" <si...@redhat.com>
> Sent: Wednesday, November 28, 2012 2:58:52 PM
> Subject: [vdsm] MTU setting according to ifcfg files.
> 
> Hi,
> 
> I am working on one of the vdsm bugs that we have and I found that
> initscripts (initscripts-9.03.34-1.el6.x86_64)
> behaviour doesn't fits our needs.
> So, I would like to raise this issue in the list.
> 
> The issue is MTU setting according to ifcfg files.
> I'll try to describe the flow below.
> 
> 1. I started with ifcfg file for the interface without MTU keyword at
> all
> and the proper interface (let say eth0) had the *default* MTU=1500
> (according to /sys/class/net/eth0/mtu).
> 2. I created a bridge with MTU=9000 on top of this interface.
> Everything went OK.
>    After I wrote MTU=9000 on ifcfg-eth0 and ifdown/ifup it, eth0 got
>    the proper MTU.
> 3. Now, I removed the bridge and deleted MTU keyword from the
> ifcfg-eth0.
>    But after ifup/ifdown the actual MTU of the eth0 stayed 9000.
>   
> The only way to change it back to 1500 (or something else) is
> explicitly set MTU in ifcfg file.
> According to Bill Nottingham it is intentional behaviour.

Right. The network should not push what not configured.

> If so, we have a problem in vdsm, because we never set MTU value
> until user ask it explicitly.
> It means that if we have interface with MTU=9000 on it just because
> once there was a bridge with such MTU
> attached to it and now we want to attach regular bridge with
> *default* MTU=1500 we have a problem.
> The only thing we can do to avoid this it's set explicitly MTU=1500
> in interface's ifcfg file.
> IMHO it's a bit ugly, but it looks like we have no choice.
> 
> As usual comments more than welcome...

For the long run have total control over our resources, and push whatever 
configuration we require... skipping distribution specific behaviour.

Regards,
Alon.
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