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Hi Luis and Daniel,

Please see inline response.

> From: Daniel Alvarez Sanchez <dalvarez at redhat.com>
> Sent: 29 September 2022 11:37
> Subject: Re: OVN BGP Agent query
>
> Hi Ihtisham and Luis,
>
> On Thu, Sep 29, 2022 at 7:42 AM Luis Tomas Bolivar <ltomasbo at redhat.com> wrote:
> > Some comments and questions inline
> >
> > On Tue, Sep 27, 2022 at 1:39 PM Ihtisham ul haq <ihtisham.uh at hotmail.com> wrote:
> > > Hi Luis,
> > >
> > > Thanks for your work on the OVN BGP Agent. We are planning
> > > to use it in our OVN deployment, but have a question regarding it.
> >
> > Great to hear! Can you share a bit more info about this environment? like
> > openstack version, target workload, etc.

We plan to use this with Yoga version. Our workload consist of enterprise users
with VMs running on Openstack and connected to their enterprise network via
transfer network(to which the customer neutron router is attached to).
And we also have public workload but with the ovn-bgp we only want to
we want to advertise the former.

> >
> > > The way our current setup with ML2/OVS works is that our customer VM IP routes
> > > are announced via the router IP(of the that customer) to the leaf switch instead of
> > > the IP of the host where the neutron BGP agent runs. And then even if the
> > > router fails over, the IP of the router stays the same and thus the BGP route
> > > doesn't need to be updated.
> >
> > Is this with Neutron Dynamic Routing? When you say Router IP, do you mean the virtual neutron router and its IP associated with the provider network? What type of IPs are you announcing with BGP? IPs on provider network or on tenant networks (or both)?

Yes, that's with Neutron DR agent, and I meant virtual neutron router with
IP from the provider network. We announce IPs of our tenant network via the
virtual routers external address.

> > If the router fails over, the route needs to be updated, doesn't it? Same IP, but exposed in the new location of the router?

Correct.

> The route to the tenant network doesn't change, ie.
> 192.168.0.0 via 172.24.4.100  (this route remains the same regardless of where 172.24.4.100 is).
> If there's L2 in the 172.24.4.0 network, the new location of 172.24.4.100 will be learnt via GARP announcement. In our case, this won't happen as we don't have L2 so we expose directly connected routes to overcome this "limitation".

Right, in our case we have a stretched L2 transfer network(mentioned above)
to which our gateway nodes and customer routers are connected to, so we can
advertise the IPs from the tenant network via the virtual router external IP
and thus the location of the router isn't relevant in case of failover as its
address will be relearned.

> In the case of Neutron Dynamic Routing, there's no assumption that everything is L3 so GARPs are needed to learn the new location.
>
> > > We see that the routes are announced by the ovn-bgp-agent via the host IP(GTW) in our
> > > switch peers. If that's the case then how do you make sure that during failover
> > > of a router, the BGP routes gets updated with the new host IP(where the router
> > > failed over to)?
> >
> > The local FRR running at each node is in charge of exposing the IPs. For the IPs on the provider network, the traffic is directly exposed where the VMs are, without having to go through the virtual router, so a router failover won't change the route.
> > In the case of VMs on tenant networks, the traffic is exposed on the node where the virtual router gateway port is associated (I suppose this is what you refer to with router IP). In the case of a failover the agent is in charge of making FRR to withdraw the exposed routes on the old node, and re-advertise them on the new router IP location
> >
> > > Can we accomplish the same route advertisement as our ML2/OVS setup, using the ovn-bgp-agent?
>
> I think this is technically possible, and perhaps you want to contribute that functionality or even help integrating the agent as a driver of Neutron Dynamic Routing?

Sounds good, our plan currently is to add this to the ovn-bgp-agent,
so we can announce our tenant routes via virtual routers external address on
a stretched L2 network, to make it work with our use case.


--
Ihtisham ul Haq
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