Conflict when creating a Floating IP

Description of problem :
When running Browbeat+Rally scenario with netcreate-boot-ping scenario (where we spawn a VM, associate a FIP and then try to ping the FIP with concurrency set to 5 and times set to 500), we observed the following in neutron server logs.

Release: Pike
Setup: ODL running in a non-cluster, non-container setup with 1 controller and 32 compute nodes.

For the initial few VMs, rally plugin was able to create FIP successfully. But from 40th VM onwards there was a failure in FloatingIP creation.

On looking at Neutron logs, we could see "conflict when trying to complete your request" errors.

2017-12-01 11:58:54.454 948420 INFO neutron.api.v2.resource [req-82bbdf57-a9ab-410e-9440-fde78d299b36 4f3065ec9a8b40ab8291b285cdd33fdd f505a5e332c5463c92ac74c48badc286 - default default] create failed (client error): There was a conflict when trying to complete your request.
2017-12-01 11:58:54.454 948420 INFO neutron.wsgi [req-82bbdf57-a9ab-410e-9440-fde78d299b36 4f3065ec9a8b40ab8291b285cdd33fdd f505a5e332c5463c92ac74c48badc286 - default default] 172.16.0.26 "POST /v2.0/routers.json HTTP/1.1" status: 409 len: 345 time: 2.6300662

The test suite is failing as its unable to create a FloatingIP.