For Neutron to properly connect DHCP namespace interfaces to the appropriate network bridge, the DHCP agent on the controller node must be configured to use the Linux bridge interface driver.
On the controller node, update the interface_driver configuration option in the Neutron DHCP agent configuration file at /etc/neutron/dhcp_agent.ini to use the Linux bridge interface driver:
[DEFAULT]
...
interface_driver = linuxbridge
The interface driver will vary based on the plugin agent in use on the node hosting the DHCP agent.