查看 neutron-openvswitch-agent 状态为up
systemctl status neutron-openvswitch-agent.service

● neutron-openvswitch-agent.service - OpenStack Neutron Open vSwitch Agent
   Loaded: loaded (/usr/lib/systemd/system/neutron-openvswitch-agent.service; enabled; vendor preset: disabled)
   Active: active (running) since Wed 2017-12-06 14:10:04 CST; 57s ago

查看 neutron agent-list 状态为down
neutron agent-list

| 96a50925-7a18-4093-b43a-0a10d1c702e0 | Open vSwitch agent | CT31 |                   | xxx   | True           | neutron-openvswitch-agent |

查看 neutron-openvswitch-agent 日志 /var/log/neutron/openvswitch-agent.log

2017-12-06 14:20:19.830 25057 ERROR ryu.lib.hub [-] hub: uncaught exception: Traceback (most recent call last):
  File "/usr/lib/python2.7/site-packages/ryu/lib/hub.py", line 54, in _launch
    return func(*args, **kwargs)
  File "/usr/lib/python2.7/site-packages/ryu/controller/controller.py", line 97, in __call__
    self.ofp_ssl_listen_port)
  File "/usr/lib/python2.7/site-packages/ryu/controller/controller.py", line 120, in server_loop
    datapath_connection_factory)
  File "/usr/lib/python2.7/site-packages/ryu/lib/hub.py", line 118, in __init__
    self.server = eventlet.listen(listen_info)
  File "/usr/lib/python2.7/site-packages/eventlet/convenience.py", line 43, in listen
    sock.bind(addr)
  File "/usr/lib64/python2.7/socket.py", line 224, in meth
    return getattr(self._sock,name)(*args)
error: [Errno 98] Address already in use

可以看到 ryu 的端口被占用了，将占用 ryu 端口的进程干掉再重启服务即可。 6633 为 ryu 默认端口。
netstat -ntlp | grep 6633

tcp        0      0 127.0.0.1:6640          0.0.0.0:*               LISTEN      19183/ovsdb-server 

kill -9 19183
systemctl restart neutron-openvswitch-agent