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 > [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
