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There is an undocumented process to reserve IP addresses for use by DHCP servers. It works by creating port with the desired IP and a device owner of reserved_dhcp_port."

openstack port create dhcp-port1 --network <network UUID> --fixed-ip subnet=<subnet UUID>,ip-address=192.0.2.2 --device reserved_dhcp_port

You can either create the port prior to enabling DHCP on the subnet or delete an existing DHCP port after creating the reserved port. In either case, Neutron should utilize the reserved port on subsequent DHCP agent creation.

For understanding from the code have a look at https://github.com/openstack/neutron/blob/master/neutron/agent/linux/dhcp.py#L1489

This will give you a more concrete answer.

I hope that helps !!