Controller/Compute nodes : neutron + ovs agent
vm 啓動後會發送dhcp request, 路徑是：
(Compute node)vm tapxxx->ovs bridge(br-int)->ovs bridge(br-eth0)-> phy-nic --------->(Controller node)phy-nic->ovs bridge(br-int)->ovs bridge(br-eth1)->dhcp tapxxx
dhcp server 按照原路徑返回response
