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Just had an (in my sense) obscure situation.

I have a Xen Server with bridged networking. Everything works fine since month. A while ago i configuresd a second bridge. only some DomUs get an channel on this bridge - my Dom0 doesn't need to / should'nt use this bridge. So just 5 minutes ago while rebooting the xen host (because of an other problem with the UPS) i decided to removed the fixed ip from the the interface of the Dom0 which belongs to the second bridge. So after reboot i noticed that none of the interfaces on the second bridge is available. I couldn't find a problem. Everything was just like before the reboot, except the interface of the Dom0 had no IP address.

After a while i tried to give the DomO interface of this bridge an IP again and ... BOOM ... everything is up and running again!

WTF? Why is it important to have the interface of a bridge configured in the Dom0? Even when confiugured 'wrong' (complitely different netowkr settings as the network really hanging on the bridge) everythjing works fine ... I don't get it. Could please someone explain?

Tnaks a lot!

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I don't understand you description exactly, but having some experience with Xen tells me that nearly everything depends on the Dom0 - all the devices are in fact running through it, so if you disable a bridge interface in Dom0, it may be like turning it off. Xen is only the hypervisor - it allows computer to run multiple machines inside, but Dom0 does all the rest of the work - including whole device operations. For every device read/write on any bridged/shared/virtual device in DomU, this goes back and execute in Dom0 to the physical hardware (there can be exceptions to this if you assign certain HW directly to the DomUs). Now for this one, not having IP address may turn the bridge down, effectively breaking your DomU's networking.

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It's been a while, but i'll try to recall: The question is: Why does a brdige need to have a IP-Address in dom0? Why can't i just leave the bridge (here: eth0) unconfigured in dom0 and only add the 'real' interface (here: peth0) and the vifs oft the domUs (here: vif5.1, vif6.1, ...) to the bridge? – Jan 14 '11 at 16:43
I've found a packet-flow explanation for xen at… and you're right - there should be no need for bridge interface itself to have an IP. – Qiqi Jan 15 '11 at 3:33

Sounds like the configuring of an IP to your Dom0-interface made the interface go "up" (in your OS).

What was the status bevore that?

Did you try a simple "ifconfig INTERFACE up" without setting an IP to it?

The setup of an interface without IP that should be "up" after reboot seems to be different on every flavour (Debian/SuSE/RedHat/Slackware/...) of Linux.

I stumbled across that many times.

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I'm sorry - it's too long ago to remember the details, but: I think i tried the 'ifconfig $INTERFACE up', but i'm not sure about it. The problem was, that the interface needed to have an IP assigned - "up" alone was not enough. But as i said: to long ago ... – Apr 17 '11 at 8:34

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