Starting with the Liberty release, OpenStack Networking includes a
pluggable interface for the IP Address Management (IPAM) function. This
interface creates a driver framework for the allocation and
de-allocation of subnets and IP addresses, enabling the integration of
alternate IPAM implementations or third-party IP Address Management
systems.

The basics

In Liberty and Mitaka, the IPAM implementation within OpenStack
Networking provided a pluggable and non-pluggable flavor. As of Newton,
the non-pluggable flavor is no longer available. Instead, it is
completely replaced with a reference driver implementation of the
pluggable framework. All data will be automatically migrated during the
upgrade process, unless you have previously configured a pluggable IPAM
driver. In that case, no migration is necessary.

To configure a driver other than the reference driver, specify it in
the neutron.conf file. Do this after the migration is
complete. There is no need to specify any value if you wish to use the
reference driver.

ipam_driver = ipam-driver-name

There is no need to specify any value if you wish to use the
reference driver, though specifying internal will
explicitly choose the reference driver. The documentation for any
alternate drivers will include the value to use when specifying that
driver.

Known limitations

• The driver interface is designed to allow separate drivers for each
  subnet pool. However, the current implementation allows only a single
  IPAM driver system-wide.
• Third-party drivers must provide their own migration mechanisms to
  convert existing OpenStack installations to their IPAM.