This bug was initially created as a copy of Bug #2314429

I am copying this bug because:

We need to add a KCS explaining the failing scenario (MTU mismatch), how to deal with it (either fix mismatch or enable the flag in Tripleo templates). The KCS will apply to all z-releases for 17.x. We may later release async that would no longer require the KCS.

Since this behavior is currently different for ml2/ovs (emitting need frag packets) and ml2/ovn (not doing it by default), this should also be documented in migration guide for ovs->ovn for 17. Namely, suggesting that the flag for Tripleo should be set to True during migration.

Description of problem:

Enabling the "ovn_emit_need_to_frag" configuration option solves a gap between ML2/OVN and ML2/OVS but, it was left disabled by default because of a kernel requirement [0]. Now it seems that we could consider enabling this in the product for newer versions of OSP as the kernel requirements are no longer a problem.

I am opening this BZ as a request from a discussion had about a customer use-case on slack, see the internal discussion for more information: https://redhat-internal.slack.com/archives/C046JULBVJ7/p1727170946243269

[0] This feature requires kernel version >= 5.2 (upstream) before being enabled. One could run the following command to verify it before enabling in our product:

$ ovs-appctl -t ovs-vswitchd dpif/show-dp-features br-int

If the output contains: "Check pkt length action: Yes" means it is supported and can be enabled.

Enabling it on a non-supported kernel version will lead to critical performance degradation.

Version-Release number of selected component (if applicable):
17.1.3