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[Openstack] ARP packets not sent during migration

On 03/20/2018 12:40 PM, Ramon Orru wrote:
> Hello everybody, I'm running a fresh queens cluster. I'm using bridges 
> to support networking. I'm facing an issue when an instance is live 
> migrated.
> Suppose we have an instance running with an interface on vlan XXX, and 
> we want to migrate it to compute host YYY. We'll call that instance ZZZ.
> If no other instance is already running on YYY using vlan XXX, no bridge 
> called 'br-vlan.XXX at br-vlan' exists yet on YYY.
> Now, if I migrate ZZZ on YYY host, a new bridge 'br-vlan.XXX at br-vlan' 
> will be created.
> During the migration process, ZZZ become unreachable while interfaces 
> are going up on YYY (from 10 seconds to about 2 minutes).
> After some troubleshooting, we spotted the problem: bridge 
> 'br-vlan.XXX at br-vlan' is being created after gratuitous ARP packets are 
> sent from migrating machines to advise other devices about new position.
> The result is: no other device can reach the fresh migrated machine 
> until ARP table becomes stale.
> This does not happen when an instance with an interface on same vlan is 
> already runnning on destination host ('br-vlan.XXX at br-vlan' is already 
> up and running, and ARPs can be sent flawlessly).
> Any idea of how to get rid of this? I think it's very unlikely that I'm 
> the first to face this problem, but i didn't manage to find additional 
> info on this strange behaviour.
> Thanks in advance.

Just to clarify - are you talking about connectivity to the floating IP 
or just on the VLAN itself, i.e. the instance is directly connected to 
the VLAN.  I'm thinking it's the latter, which would mean it's 
libvirt(?) sending the gARPs before the bridge is up?