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

Tim Rozet trozet at redhat.com
Tue Jun 16 18:32:39 UTC 2020


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.

>
> 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.


>
> Thanks,
> Han
>
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