Below in gate logs. Completely breaks the DHCP for that network because it's trying to add an address that conflicts with one given to it via RA. Cause is the merge of d86f1b87f01c53c3e0b085086133b311e5bf3ab5 which allowed the agent to be configured with stateless v6 addresses to serve metadata correctly.
http://logs.openstack.org/12/343312/5/gate/gate-tempest-dsvm-neutron-full-ubuntu-xenial/c11b933/logs/screen-q-dhcp.txt.gz?level=TRACE#_2016-09-26_21_45_40_604
2016-09-26 21:45:40.604 13605 ERROR neutron.agent.linux.utils [-] Exit code: 2; Stdin: ; Stdout: ; Stderr: RTNETLINK answers: File exists
2016-09-26 21:45:40.604 13605 ERROR neutron.agent.dhcp.agent [-] Unable to enable dhcp for 81d252a2-8207-4e8c-a286-07fb3494a3ec.
2016-09-26 21:45:40.604 13605 ERROR neutron.agent.dhcp.agent Traceback (most recent call last):
2016-09-26 21:45:40.604 13605 ERROR neutron.agent.dhcp.agent File "/opt/stack/new/neutron/neutron/agent/dhcp/agent.py", line 114, in call_driver
2016-09-26 21:45:40.604 13605 ERROR neutron.agent.dhcp.agent getattr(driver, action)(**action_kwargs)
2016-09-26 21:45:40.604 13605 ERROR neutron.agent.dhcp.agent File "/opt/stack/new/neutron/neutron/agent/linux/dhcp.py", line 212, in enable
2016-09-26 21:45:40.604 13605 ERROR neutron.agent.dhcp.agent interface_name = self.device_manager.setup(self.network)
2016-09-26 21:45:40.604 13605 ERROR neutron.agent.dhcp.agent File "/opt/stack/new/neutron/neutron/agent/linux/dhcp.py", line 1396, in setup
2016-09-26 21:45:40.604 13605 ERROR neutron.agent.dhcp.agent namespace=network.namespace)
2016-09-26 21:45:40.604 13605 ERROR neutron.agent.dhcp.agent File "/opt/stack/new/neutron/neutron/agent/linux/interface.py", line 129, in init_l3
2016-09-26 21:45:40.604 13605 ERROR neutron.agent.dhcp.agent device.addr.add(ip_cidr)
2016-09-26 21:45:40.604 13605 ERROR neutron.agent.dhcp.agent File "/opt/stack/new/neutron/neutron/agent/linux/ip_lib.py", line 577, in add
2016-09-26 21:45:40.604 13605 ERROR neutron.agent.dhcp.agent self._as_root([net.version], tuple(args))
2016-09-26 21:45:40.604 13605 ERROR neutron.agent.dhcp.agent File "/opt/stack/new/neutron/neutron/agent/linux/ip_lib.py", line 364, in _as_root
2016-09-26 21:45:40.604 13605 ERROR neutron.agent.dhcp.agent use_root_namespace=use_root_namespace)
2016-09-26 21:45:40.604 13605 ERROR neutron.agent.dhcp.agent File "/opt/stack/new/neutron/neutron/agent/linux/ip_lib.py", line 95, in _as_root
2016-09-26 21:45:40.604 13605 ERROR neutron.agent.dhcp.agent log_fail_as_error=self.log_fail_as_error)
2016-09-26 21:45:40.604 13605 ERROR neutron.agent.dhcp.agent File "/opt/stack/new/neutron/neutron/agent/linux/ip_lib.py", line 104, in _execute
2016-09-26 21:45:40.604 13605 ERROR neutron.agent.dhcp.agent log_fail_as_error=log_fail_as_error)
2016-09-26 21:45:40.604 13605 ERROR neutron.agent.dhcp.agent File "/opt/stack/new/neutron/neutron/agent/linux/utils.py", line 138, in execute
2016-09-26 21:45:40.604 13605 ERROR neutron.agent.dhcp.agent raise RuntimeError(msg)
2016-09-26 21:45:40.604 13605 ERROR neutron.agent.dhcp.agent RuntimeError: Exit code: 2; Stdin: ; Stdout: ; Stderr: RTNETLINK answers: File exists
2016-09-26 21:45:40.604 13605 ERROR neutron.agent.dhcp.agent
2016-09-26 21:45:40.604 13605 ERROR neutron.agent.dhcp.agent
The offending patch should be reverted