Comment 2 for bug 1568809

Essentially you always need to update an existing Neutron port in one of two ways:
1) Modify the "fixed_ips" list of dictionaries
2) Modify the "allowed_address_pairs" list of dictionaries

The first one is more management-clean because Neutron will then take the new IP(s) into account when you try to assign them to another port, for example. On the other hand, the current DHCP implementation will get confused by multiple IP records for the same MAC and will only offer the least recently added IP address.

The second way basically just says "let these IPs in and out of this port" and doesn't in any way modify DHCP offers nor checks whether the added IPs have already been assigned elsewhere. Have a look at the answer here: https://ask.openstack.org/en/question/84676/how-to-add-multiple-ip-inside-allowed-address-pairs/