Comment 2 for bug 1757482

Armondo I will work on providing more detail for the issue. Received the below from Kevin Benton through the mailing list so wanted to add to the bug for additional feedback.

I think you might have uncovered an edge-case that should probably be filed as a bug against Neutron. If a router interface is attached using a reference to a subnet, it always tries to use the address in the "gateway_ip" of the subnet:
https://github.com/openstack/neutron/blob/282d3da614f24a6385c63a926a48845d3f6d72a3/neutron/db/l3_db.py#L797-L798

My opinion is that Neutron probably shouldn't allow grabbing the default gateway if you aren't the owner of the subnet, but that is a fix that might not land for a while depending on their priorities.

In the meantime, I recommend that you create a neutron port as an admin on the public network using the gateway_ip of the network to represent your real gateway router. This will prevent anyone from being able to attach a router using the subnet as a reference since the gateway_ip address will already be in use.