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Re: [Xen-users] network-bridge breaks networking when eth0:1 is added

Fajar A. Nugraha wrote on Wed, 29 Apr 2009 10:45:22 +0700:

> You have an error here.
> you assign eth2 and xenbr1 IPs from the same IP subnet (
> and are both on, yet they're not on the
> same physical network (not on the same bridge, and not connected to
> the same switch/vlan).
> If the interfaces are on different physical network, you need to
> assign different IP subnet.  I believe this is the root cause of
> problem on your setup : basic TCP/IP networking.

Do I? I think it's not disallowed, but it creates extra problems. Certainly. 
In the end it isn't worth it. If I had experience with three NICs I had known 
that right-away. But the other machines are 2 NIC and I "automatically" 
assigned different nets. Just not this time, because it didn't seem necessary.

> > I now realize that my best option is
> > probably to use a different subnet each for eth1 and eth2
> Correct.


> This would mean that dom0 and domU can communicate (through bridging
> on xenbr1). But this also mean domU will NOT be able to communicate to
> whatever machine connected to dom0's eth2 (a4 machine?). If you want
> it you need to :
> - Setup dom0 as router, OR
> - create another bridge for eth2 (say xenbr2) and add another vif on
> domU connected to that bridge.

I have done the latter. As you may remember this machine will eventually not 
be the target (as in these tests), but the machine pulling the data from domUs 
on other machines. So, I go out via xenbr1 to machine a1 and via xenbr2 to 
machine a2. I don't need to go in the other direction, at least not to domUs.
So, there is no need for bridging on this machine in theory. Just for the case 
that I may want it in the future nevertheless I have it now changed to have 
three bridges, though. Once you know how this works it's really easy. I have 
also reconfigured one other machine to use the bridge setup instead of relying 
on Xen's network-bridge. One problem I encountered there (and I think also 
with c4) is that I had to reboot to get the complete new network setup 
working. eth0/peth0 didn't want to go away and thus the new bridge could not 
be created. I couldn't brctl delbr nor ifdown the devices.

Again, thanks for the help, I'm now set to continue on my own with the two 
other machines.


Kai Schätzl, Berlin, Germany
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