[libvirt] networking with two hosts

Radek Hladik radek at eadresa.cz
Thu Feb 26 20:08:26 UTC 2009

Andreas Rittershofer napsal(a):
> Am 26.02.2009 um 20:26 schrieb Radek Hladik:
>> Andreas Rittershofer napsal(a):
>>> My problem is:
>>> 1.) VM1 running on host A must have network access to VM3 when VM3 is 
>>> running on host B by default _and_
>>> 2.) when VM3 is running on host A because host B failed.
>>> In some of my configurations 1.) worked but 2.) not, in some other 
>>> configurations 2.) worked but 1.) not.
>>> I must have missed some detail ...
>> I do not see the problem. With the bridged scenario every VM has 
>> direct access to the physical network and it is not important on which 
>> host it runs. If the VMs are running on the same host, their ethernet 
>> frames will just "turn around" in the bridge interface and if they are 
>> running on different hosts they will go thru the real ethernet.
>> State 1)
>> A: VM1
>> B: VM3
>> Eth. frames will go like this: 
>> VM1->BrA->PhysCardA->Ethernet->PhysCardB->BrB->VM3
>> State 2)
>> A: VM1+VM3
>> B: x
>> Eth. frames will go like this:
>> VM1->BrA->VM3
> NOW I've got it!
> The errors in my configuration were two different network configurations 
> on host A and on host B - I always thought that the network on host A 
> and the network on host B must have different IP addresses. Now there is 
> only on network configuration for host A _and_ host B and everything is 
> fine!
> Thank you very much for your help!
> mfg ar

Glad to hear its working! All the involved machines (real and 
virtualized) share the same ethernet segment so they need to have IPs 
from the same range. You can even assign IPs for the VMs via DHCP 
server, etc.
Or to be more correct, you can have different network IP ranges, but 
then you need to route the traffic somehow...


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