[ovs-discuss] [OVN] running bfd on ecmp routes?

Numan Siddique numans at ovn.org
Wed Jun 17 10:43:29 UTC 2020

On Wed, Jun 17, 2020 at 12:50 AM Han Zhou <zhouhan at gmail.com> wrote:

> On Tue, Jun 16, 2020 at 11:32 AM Tim Rozet <trozet at redhat.com> wrote:
>> Thanks Han. See inline.
>> Tim Rozet
>> Red Hat CTO Networking Team
>> On Tue, Jun 16, 2020 at 1:45 PM Han Zhou <zhouhan at gmail.com> wrote:
>>> On Mon, Jun 15, 2020 at 7:22 AM Tim Rozet <trozet at redhat.com> wrote:
>>>> Hi All,
>>>> While looking into using ecmp routes for an OVN router I noticed there
>>>> is no support for BFD on these routes. Would it be possible to add this
>>>> capability? I would like the next hop to be removed from the openflow group
>>>> if BFD detection for that next hop goes down. My routes in this case would
>>>> be on a GR for N/S external next hop and not going across a tunnel as it
>>>> egresses.
>>>> Thanks,
>>>> Tim Rozet
>>>> Red Hat CTO Networking Team
>>>> Hi Tim,
>>> Thanks for bringing this up. Yes, it is desirable to have BFD support
>>> for OVN routers. Here are my thoughts.
>>> In general, OVN routers are distributed. It is not easy to tell which
>>> node should be responsible for the BFD session, especially, to handle the
>>> response packets. Even if we managed to implement this, the node detects
>>> the failure needs to populate the information to central SB DB, so that the
>>> information is distributed to all nodes, to make the distributed route
>>> updated.
>> Right in a distributed case it would mean the BFD endpoint would be under
>> the network managed by OVN, and therefore reside on the same node where the
>> port for that endpoint resides. In the ovn-kubernetes context, it is a pod
>> running on a node connected to the DR.
> Yes, this may be the typical case. However, there can be more scenarios,
> since there is no limit for what the nexthop can be in OVN routes. It can
> be an IP of a OVN port which is straightforward. It can also be an IP of a
> nested workload which is under the OVN managed network but not directly
> known by OVN (maybe learned through ARP). The nexthop can also be on
> external networks reachable through distributed gateway ports (instead of
> GR), in which case the routes are distributed and it requires resolving the
> output port to figure out that the BFD session should be running through
> the gateway node. But I agree that all these should be doable, although it
> may introduce some complexity. In addition, for distributed routers, BFD is
> not necessarily faster than an external monitoring mechanism, because the
> updates to the route would anyway need to go through the central DB (so
> that it can be enforced on all nodes in the distributed manner).

Maybe we can extend the current service monitor implementation to also
include BFD ? And detect any failures.

Whether the nexthop is known to OVN or is outside of OVN subsystem,
ovn-controller can inject the BFD packet to the router pipeline and this
packet would be routed and get delivered
to the endpoint handling the nexthop. If we take this approach, OVN doesn't
need to know what is the OVS interface to use to enable BFD if the
interface connected to the nexthop endpoint.

Right now, ovn-controller creates the OVS tunnel interfaces and it can
easily configure BFD on these. But generally, OVS interfaces for VMs/PODs
etc are created by external entities like - OpenStack Nova, ovn-kubernetes
and probably its not a good idea for ovn-controller to enable the BFD by
running equivalent of - "ovs-vsctl set interface <interface_name>

Any thoughts ?


>>> In your particular case, it may be easier, since the gateway router is
>>> physically located on a single node. ovn-controller on the GR node can
>>> maintain BFD session with the nexthops. If a session is down,
>>> ovn-controller may take action locally to enforce the change locally.
>> Yeah for the external network case this makes sense. I went ahead and
>> filed a BZ:
>> https://bugzilla.redhat.com/show_bug.cgi?id=1847570
>>> For both cases, more details may need to be sorted out.
>>> Alternatively, it shouldn't be hard to have an external monitoring
>>> service/agent that talks BFD with the nexthops, and react on the session
>>> status changes by updating ECMP routes in OVN NB.
>> Yeah I have a workaround plan to do this for now, using a networking
>> health check and signaling from K8S. The problem is this is much slower
>> than using real BFD, but it is better than nothing.
> Great. Is there any design doc or POC? (or if there is a plan to share
> when ready). Thanks!
>>> Thanks,
>>> Han
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