Comment 0 for bug 1609296

The neutron DR agent only advertise floating IP routes as destination(floating IP/32) - next_hop(gateway IP).
Such routes may cause link unreachable in a layer 3 isolated multi-AZ network environment.

For instances:
ISP1 - DC1
ISP2 - DC2
ISP3 - DC3

For the floating IPs, we let all the ISP to advise the IP range, such as 172.16.1.0/24.
And each DC is layer 3 isolated in a separate AZ managed by one neutron deployment.

So for a floating IP (172.16.1.10) bgp route will be:
+-----------------+--------------+
| destination | next_hop |
+-----------------+--------------+
| 172.16.1.10/32 | 172.16.1.2 |
+-----------------+--------------+

We use more than one DR agent to advise that route to all DC physic device (router).
Then assuming that 172.16.1.10 is served at DC1. Some incoming traffic from DC2 or DC3 now does
not know how to route the 172.16.1.10, because the 172.16.1.2 is stranger to them, aka link unreachable.

So advising the gateway routes may be needed in such scenario.