[Bug 1905933] Re: Incorrect ARP processing when enable_distributed_floating_ip=True

[Frode Nordahl]

We have triggered rebuilds now and the arm builds appear to be
successful.

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https://bugs.launchpad.net/bugs/1905933

Title:
  Incorrect ARP processing when enable_distributed_floating_ip=True

Status in ovn package in Ubuntu:
  Fix Released
Status in ovn source package in Focal:
  Fix Committed
Status in ovn source package in Groovy:
  Fix Released
Status in ovn source package in Hirsute:
  Fix Released

Bug description:
  [Impact]
  Enabling `enable-distributed-floating-ip` on a cloud with OVN 20.03 results in loss of external connectivity for instances with floating IPs.

  [Test Case]
  Launch two instances and assign floating IPs to them. Toggle the `enable-distributed-floating-ip` configuration option and attempt to access a IP address on the internet that is not reachable in the external network L2 broadcast domain.

  Observe as the instances will attempt to reach the IP by obtaining
  it's MAC address through ARP resolution directly rather than applying
  L3 routing.

  The functional test gate of the neutron-api-plugin-ovn charm may be
  useful for verification.

  [Regression Potential]
  We have cherry-picked a patch from upstream that reverts the change that introduced the erratic behavior in its entirety. The optimization has later been replaced by a new set of patches which is available in newer versions. As such the regression potential is minimal.

  [Original Bug Report]
  In a focal-ussuri deployment when enabling `enable-distributed-floating-ip` traffic from instances with FIPs should exit the HV directly and not go through a gateway chassis.

  However due to a bug each HV will attempt to do ARP processing locally
  even for IP addresses not in the external network CIDR.

  This results in loss of connectivity for instances with FIPs.

  The issue is not present in Groovy with OVN 20.06 and I suspect the issue is fixed by this commit:
  https://github.com/ovn-org/ovn/commit/d9ed450713eda62af1bec5009694b2d206c9f435

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