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Our purpose is to provide uninterrupted web services with low cost in OPENSTACK. Due to all instances traffic through Nova-Network host in a basic deployment of NOVA . And it's hard to do HA+LB for Nova-Network. As I know it will be very hard to implement. After read "Bootstrapping OpenStack Clouds" doc writed by DELL....The fault zone concept it much easier and cheaper.
There're three diagrams .... I.Two zones topology , share nothing between each zones. But combine two instance network in same network segment. Zone1 using 192.168.1.0/25, Zone2 using 192.168.1.128/25 . We do not using floating ip. Let instance route from Core Network Switch directly.