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Hello Gary:

If you have 2 VMs in the same network, both without FIPs and one is working
but not the other, I would just compare 1:1 the Neutron and OVN resources
of both ports (I guess both VMs have one single port). I would start with
the OVN NAT registers. You should also check the internal VM routing table.

Apart from that, you should also trace the VM traffic, to know where it is
dropped. Maybe the traffic is sent correctly outside the GW port but never
gets back (in that case, check you underlying network configuration). Or,
as you commented, the SNAT is not working for this specific port.

Regards.

On Tue, Jun 27, 2023 at 1:38 PM Gary Molenkamp <molenkam at uwo.ca> wrote:

> Good morning,   I'm having a problem with snat routing under OVN but I'm
> not sure if something is mis-configured or just my understanding of how
> OVN is architected is wrong.
>
> I've built a Zed cloud, since upgraded to Antelope, using the Neutron
> Manual install method here:
> https://docs.openstack.org/neutron/latest/install/ovn/manual_install.html
> I'm using a multi-tenent configuration using geneve and the flat
> provider network is present on each hypervisor. Each hypervisor is
> connected to the physical provider network, along with the tenent
> network and is tagged as an external chassis under OVN.
>          br-int exists, as does br-provider
>          ovs-vsctl set open .
> external-ids:ovn-cms-options=enable-chassis-as-gw
>
> For most cases, distributed FIP based connectivity is working without
> issue, but I'm having an issue where VMs without a FIP are not always
> able to use the SNAT services of the tenent network router.
> Scenario:
>      Internal network named cs3319:  with subnet 172.31.100.0/23
>      Has a router named cs3319_router with external gateway set (snat
> enabled)
>
>      This network has 3 vms:
>          - #1 has a FIP and can be accessed externally
>          - #2 has no FIP, can be accessed via VM1 and can access
> external resources via SNAT  (ie OS repos, DNS, etc)
>          - #3 has no FIP, can be accessed via VM1 but has no external
> SNAT connectivity
>
>  From what I can tell,  the chassis config is correct, compute05 is the
> hypervisor and the faulty VM has a port binding on this hypervisor:
>
> ovn-sbctl show
> ...
> Chassis "8e0fa17c-e480-4b60-9015-bd8833412561"
>      hostname: compute05.cloud.sci.uwo.ca
>      Encap geneve
>          ip: "192.168.0.105"
>          options: {csum="true"}
>      Port_Binding "7a5257eb-caea-45bf-b48c-620c5dff4b39"
>      Port_Binding "50e16602-78e6-429b-8c2f-e7e838ece1b4"
>      Port_Binding "f121c9f4-c3fe-4ea9-b754-a809be95a3fd"
>
> The router has the candidate gateways, and the snat set:
>
> ovn-nbctl show  92df19a7-4ebe-43ea-b233-f4e9f5a46e7c
> router 92df19a7-4ebe-43ea-b233-f4e9f5a46e7c
> (neutron-389439b5-07f8-44b6-a35b-c76651b48be5) (aka cs3319_public_router)
>      port lrp-44ae1753-845e-4822-9e3d-a41e0469e257
>          mac: "fa:16:3e:9a:db:d8"
>          networks: ["129.100.21.94/22"]
>          gateway chassis: [5c039d38-70b2-4ee6-9df1-596f82c68106
> 99facd23-ad17-4b68-a8c2-1ff6da15ac5f
> 1694116c-6d30-4c31-b5ea-0f411878316e
> 2a4bbaf9-228a-462e-8970-0cdbf59086e6 9332c61b-93e1-4a70-9547-701a014bfd98]
>      port lrp-509bba37-fa06-42d6-9210-2342045490db
>          mac: "fa:16:3e:ff:0f:3b"
>          networks: ["172.31.100.1/23"]
>      nat 11e0565a-4695-4f67-b4ee-101f1b1b9a4f
>          external ip: "129.100.21.94"
>          logical ip: "172.31.100.0/23"
>          type: "snat"
>      nat 21e4be02-d81c-46e8-8fa8-3f94edb4aed1
>          external ip: "129.100.21.87"
>          logical ip: "172.31.100.49"
>          type: "dnat_and_snat"
>
> Each network agent on the hypervisors shows the ovn controller up :
>       OVN Controller Gateway agent | compute05.cloud.sci.uwo.ca
> |                   | :-)   | UP    | ovn-controller
>
> The ovs vswitch on the hypervisor looks correct afaict and ovn ports bfd
> status are all forwarding to other hypervisors. ie:
>     Port ovn-2a4bba-0
>              Interface ovn-2a4bba-0
>                  type: geneve
>                  options: {csum="true", key=flow,
> remote_ip="192.168.0.106"}
>                  bfd_status: {diagnostic="No Diagnostic",
> flap_count="1", forwarding="true", remote_diagnostic="No Diagnostic",
> remote_state=up, state=up}
>
>
> Any advice on where to look would be appreciated.
>
> PS.  Version info:
>      Neutron 22.0.0-1
>      OVN 22.12
>
>     neutron options:
>        enable_distributed_floating_ip = true
>        ovn_l3_scheduler = leastloaded
>
>
>
> Thanks
> Gary
>
>
>
> --
> Gary Molenkamp                  Science Technology Services
> Systems/Cloud Administrator     University of Western Ontario
> molenkam at uwo.ca                  http://sts.sci.uwo.ca
> (519) 661-2111 x86882           (519) 661-3566
>
>
>
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