In my understanding, the standby and primary hostdev interface may be in different subnet. 

I'm not sure whether it is correct. Could you please help to explain? Thank you in advance.

For example, primary hostdev is connected to vf-pool with <pf='eth0'/>, while the standby is connected to NAT network with " forward dev='eth0'".

The standby interface will get ip as 192.168.122.x, but after NAT, it will be in the same subnet of the vf.  

So after the VF is unplugged, the packet will still broadcast in the same subnet, and the vm will get the packet as the standby share the same mac. Right?

Thank you!

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Best Regards,
Yalan Zhang
IRC: yalzhang