Comment 8 for bug 1496204

the real issue is that:
1. there are 3 compute nodes and 2 network nodes
2. create a DVR router, set router_gateway and add one subnet to this router
3. create some VMs in this above subnet(we assume every compute has VM running on it)
4. update router gateway_ip

now neutron-server will re-schedule this router, in the re-schedule method, it will firstly unbind_router from all l3 agents and bind them again, in large scale, it need much time to re-schedule, during the re-schedule, the VMs in compute node can't communicate with their gateway.
I think when we update router gateway ip, we should not unbind_router from l3 agents which running on compute nodes, just need to re-schedule the l3 agents on network nodes