[Rdo-list] instance ips not reusable ?

Hello, I notice that, in my icehouse testbed that runs neutron in OVS/VLAN mode, ips from the deleted instances don't seem to be reassigned to new instances, new instances always get the next available ip from the range incrementally. I am afraid this will cause problem eventually. On old releases, the ips of the deleted instances get reused pretty quickly on new instances. Do I miss some configuration option? Thanks, Xin