Comment 2 for bug 1575146

If physnet1 is "eth1", i think use can just use "ifconfig eth1" to comfirm the status of physnet1.
The port of vm should have the infomation of it's net and physnet.So, we can find the relationship between them.
Then we can make the port status which use for vm is down when the physnet1 is down or not according to the configure option in neutron.conf.

This is usefull for some user of VM who need know the packet they send shoul d send out of the host or not.