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[OpenStack Infra <2032817@xxxxxxxxxxxxxxxxxx>]

Reviewed:  https://review.opendev.org/c/openstack/neutron/+/937026
Committed: https://opendev.org/openstack/neutron/commit/d080b35782c705b105da61af3949d67832efd2c8
Submitter: "Zuul (22348)"
Branch:    master

commit d080b35782c705b105da61af3949d67832efd2c8
Author: Rodolfo Alonso Hernandez <ralonsoh@xxxxxxxxxx>
Date:   Wed Dec 4 14:02:02 2024 +0000

    [OVN] Set always the GW LRP "gateway_mtu" option
    
    If the configuration option "ovn.ovn_emit_need_to_frag" is set, write
    always the "LSP.options.gateway_mtu" value, using the minimum MTU
    value of all networks connected to the router. That will limit the
    N/S traffic MTU regardless if the gateway network MTU is greater or
    less than the internal network MTU.
    
    Closes-Bug: #2032817
    Change-Id: Ia1f0c0955d2fd90c7ff6307b1bc9819721ee2f9b


** Changed in: neutron
       Status: In Progress => Fix Released

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https://bugs.launchpad.net/bugs/2032817

Title:
  [OVN] packet drops when provider network MTU exceeds tenant network
  MTU

Status in neutron:
  Fix Released

Bug description:
  At the moment, we have a provider network running 1500 MTU and then
  some tenant networks running 1450 MTU.

  We've noticed that southbound traffic from the provider network to the
  tenant network will result in packets being dropped since the MTU is
  smaller.

  So, the VM would not be able to get any traffic if the packet size >
  1450, since the provider network operates at 1500 and hands over 1500
  packets to the tap interface which gets dropped by the kernel:

  [Wed Aug 23 14:33:19 2023] tapdf9341b0-6d: dropped over-mtu packet: 1456 > 1450
  ...

  It seems that OVN should fragment packets when it notices the MTU of
  the target NAT is smaller than it's own, otherwise traffic goes into a
  nowhere.

  OVN: 23.03.0

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