Comment 0 for bug 1821299

Associating Floating IP with a port can result in duplicate Floating IPs, due to the original FIP not being removed from the SNAT namespace. This is likely specific to using DVR.

We're creating a Heat stack containing, among other things, a Floating IP and a Port.

  head_a_floating_ip:
    properties:
      floating_network_id: cf0a6df9-b533-457b-8cd7-0336f0649213
      port_id:
        get_resource: head_a_external_port
    type: OS::Neutron::FloatingIP
  head_a_external_port:
    properties:
      network_id:
        get_resource: external_net
      port_security_enabled: true
      replacement_policy: AUTO
      security_groups:
      - get_resource: head_sec_group
    type: OS::Neutron::Port

During this initial stack creation, we are not creating any VMs. So, the port is not attached to any device.

It looks like because of those two lines in floating-ip definition
  port_id:
        get_resource: head_a_external_port
after the initial stack creation the Floating IP gets allocated in a SNAT namespace of one of the hypervisors, and starts to respond to ARP requests.

However, as soon as we update this stack, adding a VM, and making the above mentioned port part of a VM, something weird happens. As expected, Neutron then allocates that FIP on the hypervisor hosting the VM (as expected, we're running DVR), however Neutron fails to remove the FIP it had created initially in the SNAT namespace, after the initial stack creation.

This results in FIP being present on two different hypervisors, causing duplicate ARP replies (one MAC being in the SNAT namespace, the other in the floating ip namespace), and obvious connectivity issues.

Note that the issues does not appear if the initial FIP happens to land in the SNAT namespace of the same hypervisors which will later (after stack update) also host the VM.

Simple, confirmed, workaround is to NOT include those two lines during the initial heat stack creation, and only include them in the stack update during which we add the VM.
  port_id:
        get_resource: head_a_external_port
Not including those lines initially in the stack results in Neutron not allocating the FIP anywhere.