Neutron ML2/OVN packet fragmentation problems

Solution Verified - Updated

Environment

  • Red Hat OpenStack Platform 16
  • Red Hat OpenStack Platform 17
  • Red Hat OpenStack Services on OpenShift 18

Issue

  • Packets are silently dropped by OVN if they are bigger than network MTU
  • Packet drops when provider network MTU exceeds tenant network MTU
  • OVN does not correctly fragment IPv4 packets when the MTU of the target network is smaller than the MTU of the source network

Resolution

  • The core OVN implementation did not support fragmentation of East/West traffic using an OVN router between two private networks. Jira Issue: This content is not included.This content is not included.https://issues.redhat.com/browse/FDP-524
    • The fix for this issue is provided from ovn24.09-24.09.0-33.el9fdp(RHBA-2024:8555).
    • For OVN24.03, update to ovn24.03-24.03.5-73.el9fdp(RHBA-2025:8363) or later.
  • The z release for RHOSO18 and RHOSP17.1 are below:
    • RHOSO18: RHOSO18.0.10(FR3) or later
    • RHOSP17.1: RHOSO17.1.8 or later
  • For North/South traffic flows, there are two cases.
    • Case: MTU external < MTU internal scenario
      [ovn]/ovn_emit_need_to_frag configuration option (OVNEmitNeedToFrag THT parameter for RHOSP; should be set via customServiceConfig for neutron-operator for RHOSO) could be used on newer kernels to make OVN emit correct ICMP messages.
    • Case: MTU external > MTU internal scenario.
      RHOSP17.1: The fixed package is openstack-neutron-18.6.1-17.1.20250408181013.85ff760.el9ost or later.
      RHOSO18.0: The fixed package is RHOSO18.0.5.
  • NOTE: To make the fix effective, the affected routers need to be recreated.

Root Cause

For some North-South and East-West flows OVN silently drops packets if they are bigger than outbound network's MTU.

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