Bug 2195931

If your Red Hat OpenStack Platform (RHOSP) deployment uses Cisco ACI or other third-party ML2 mechanism drivers that rely on `neutron-dhcp-agent`, you must add `DhcpAgentNotification: true` to your custom heat templates before updating to RHOSP 16.2.5. + In RHOSP 16.2.5, DHCP agent notification is now disabled by default. This configuration change facilitates easier deployment of RHOSP with the ML2/OVN mechanism driver, because RHOSP ML2/OVN deployments do not use the DHCP agent. The heat templates used in ML2/OVS deployments were also modified to provide uninterrupted use of DHCP services in ML2/OVS deployments. + If your deployment uses the OVN or OVS ML2 mechanism driver, your updated deployment will not be affected. + Some third-party mechanism drivers use DHCP and rely on the DHCP notifications. If your environment uses one of these third-party mechanism drivers and you update to RHOSP 16.2.5 without enabling DHCP notification (`DhcpAgentNotification: true`), your updated RHOSP deployment is subject to the following issues: * Network namespaces and `dnsmasq` processes are not created for new networks and ports. * DHCP and DNS are not served to VM instances on the new networks. * New instances fail to launch on existing networks with an error message that the instance cannot be scheduled.