On 9/5/25 7:23 AM, Han Zhou wrote: > On Thu, Sep 4, 2025 at 10:22 PM Han Zhou <[email protected]> wrote: >> >> >> >> On Thu, Sep 4, 2025 at 4:15 PM Dumitru Ceara <[email protected]> wrote: >>> >>> On 9/4/25 8:51 PM, Han Zhou wrote: >>>> Hi everyone, >>>> >>> >>> Hi, >>> >>> I'm adding the rest of the ovn-kubernetes maintainers to the list of >>> recipients of this email too. >>> >>>> There is an issue raised in ovn-k8s community [0] due to a regression >>>> introduced by OVN commit [1]. The commit breaks the MEG (Multiple >>>> External Gateways) feature. The feature requires that src-route is >>>> preferred over dst-route. >>>> >>>> It was considered risky to change the default behavior with commit > [1], >>>> and so it had been hanging around for several months asking for >>>> feedback. Unfortunately this didn't get attention until now. >>>> >>>> The old behavior of OVN (before commit [1]): longest prefix length > route >>>> is preferred, regardless of src or dst. If the prefix length is the >>>> same, prefer dst over src route. This was essentially wrong because > src >>>> and dst IP are different fields and it is not reasonable to compare > the >>>> prefix length between them. This old behavior leads to unreasonable >>>> behavior for ovn-k8s central mode cluster router east-west traffic > when >>>> there are different node-subnet prefix lengths across nodes. The MEG >>>> feature of ovn-k8s happened to work because the src routes added for >>>> that feature were all /32. What the feature really requires is to > prefer >>>> src routes over dst routes. After commit [1], the unpredictable east- >>>> west traffic routing problem in the central mode cluster router is >>>> resolved, but the MEG feature is broken. >>>> >>>> Now the problem is, ovn-k8s' central mode cluster router requires dst >>>> routes over src routes, while the MEG feature requires src routes over >>>> dst routes. For ovn-k8s IC mode, the cluster router src routes are not >>>> required any more, so they can be removed, and the central mode is > going >>>> to be deprecated anyway. So ovn-k8s would prefer src-over-dst. >>>> >>>> At the same time, we are releasing OVN 25.09 tomorrow. Based on the >>>> above information, we have below options: >>>> >>>> Option 1: revert the commit [1] before the 25.09 release. This is the >>>> easiest, but IMHO it is not the right thing to do since we will go > back >>>> to the *wrong* behavior and continue encouraging the bad design of > CMS. >>>> >>> >>> My vote goes to Option 1 for now. The only reason I acked [1] >>> originally was because there was _no_ objection from ovn-kubernetes >>> maintainers and we were operating under the impression that _no_ >>> ovn-kubernetes features get broken by the change in behavior (see >>> original discussion >>> > https://patchwork.ozlabs.org/project/ovn/patch/[email protected]/#3420641 > ). >>> >>> That turned out to be a wrong assumption. In my opinion, we cannot >>> accept a behavior change the knowingly breaks users. So we should >>> revert [1]. >>> >>>> Option 2: keep the current behavior of OVN as default behavior in > 25.09 >>>> release. We can add an option to let users change the behavior so that >>>> src route is preferred over dst route. In particular for ovn-k8s, it > can >>>> be configured to src-over-dst so that the MEG feature can be fixed, > but >>>> it should only be used for IC mode (and at the same time remove the > src >>>> routes for IC mode). For central mode probably there is no user for > the >>>> MEG feature and central mode will be deprecated, so we assume it is > not >>>> a problem to keep the dst-over-src behavior. This option can be added >>>> after the 25.09 release as a bug fix (backport to 25.09). >>>> >>> >>> However, I don't think I'd oppose backporting a new feature like this. >>> But it would have to be an opt-in feature, not an opt-out as suggested > here. >>> >>> That is, I think we should revert [1] and add an opt-in knob or option >>> to change behavior. We can backport this knob/option to 25.09.z. >>> >>> We already broke ovn-kubernetes [0] why would we risk breaking other >>> CMSs too? >>> >>>> Option 3: like option 2, making the behavior configurable, but default >>>> to src-over-dst. The problems of this option are: >>>> - it would immediately break ovn-k8s central mode east-west traffic >>>> because the central mode relies on src-routes having lower priority. >>>> - we'd better make this change before the release, which is a little >>>> risky and may delay the release. Otherwise, we would end up with >>>> changing the default behavior again after the release. >>>> >>>> I personally prefer option 2. >>>> Thanks folks for the discussion. Opinions are welcome! >>>> >>>> Best, >>>> Han >>>> >>> >>> Regards, >>> Dumitru >>> >> >> Thanks Dumitru for the feedback. I sent a patch to revert the commit [1]. >> If there are no other opinions, we may merge it and backport to 25.09 > before the release. >
Hi Han, Ales and Surya shared their opinions [0] [1]. I went ahead and merged the revert. Would you have time to work on making the routing behavior configurable (default as it is on 25.03 and opt-in)? As discussed we can backport that to branch-25.09 after the release. Regards, Dumitru [0] https://mail.openvswitch.org/pipermail/ovs-dev/2025-September/426008.html [1] https://mail.openvswitch.org/pipermail/ovs-dev/2025-September/426012.html > Sorry, forgot the link to the patch: > https://patchwork.ozlabs.org/project/ovn/patch/[email protected]/ > >> >> Best, >> Han >> >>>> [0] https://cloud-native.slack.com/archives/C08452HR8V6/ >>>> p1756994862428589 < > https://cloud-native.slack.com/archives/C08452HR8V6/ >>>> p1756994862428589> >>>> [1] https://github.com/ovn-org/ovn/ >>>> commit/27cc274e66acd9e0ed13525f9ea2597804107348 < > https://github.com/ovn- >>>> org/ovn/commit/27cc274e66acd9e0ed13525f9ea2597804107348> > >> > _______________________________________________ dev mailing list [email protected] https://mail.openvswitch.org/mailman/listinfo/ovs-dev
