Comment 6 for bug 1185518

Implications for IPAM are:
1) DHCP agent should be notified so that when lease expires the default route is removed from running VMs
2) If a network:router_interface port is using the gateway_ip address we need to understand whether we want to raise (probably not), or add the gateway ip to the allocation pool (probably not either).

When creating a subnet you can submit a null gateway with --no-gateway. The CLI has no mechanism for explicitly specifying null values, as you say.