Comment 2 for bug 1609296

@Na Zhu (nazhu), thank you for your reply.
We need the following routes to physical device (router):
+-----------------+--------------+
| destination | next_hop |
+-----------------+--------------+
| 172.16.1.10/32 | 172.16.1.2 |
| 172.16.1.2/32 | new_ip[1] | (new, the routes for gateway IPs)
+-----------------+--------------+

[1] the new_ip could be:
(1) l3_agent host br-ex related "NIC IP", this may also need to add some routes which destination is the "NIC IP" to physic router.
(2) the physical router corresponding port IP to another DC.