[Rdo-list] instances stuck in BUILD state

Hello, I am having a problem with my icehouse testbed, which has one controller, one network node (OVS/VLAN) and ~50 compute nodes. It runs fine for some time, but then recently, only a small portion of new instances can start, most of the new instances are stuck in BUILD forever, without an IP assigned. The scheduler log shows the scheduler has chosen the compute node to start the instance, but the neutron server log shows no sign of allocating MAC/IP addresses for it, I suspect the neutron server didn't get the request for some reason. Any suggestions on how to debug this? Thanks, Xin