Comment 3 for bug 1381082

Jacek,
I believe that the API expects you do do:
    - del_network_ipam(ipam)
    - add_network_ipam(ipam, <new value>)

In order to replace the existing value.

The code that is being used by the neutron plugin is:
https://github.com/Juniper/contrail-controller/blob/master/src/config/vnc_openstack/vnc_openstack/neutron_plugin_db.py

method: subnet_create.
Look at line 2286: If there is an existing link between the virtual-network and the IPAM object the code is modifying the list of subnets associated with the link before doing the update.

I agree that the API is not very clear in this case. Ajay, please chime in.