[xena][neutron][ovn] Follow up to BGP with OVN PTG discussions

Dan Sneddon dsneddon at redhat.com
Thu May 13 19:48:23 UTC 2021

Thank you all who attended the discussions at the Xena PTG regarding BGP 
dynamic routing with Neutron using OVN. Here's a brief summary of the 
important points covered, and some background information.

Red Hat has begun gathering a team of engineers to add OpenStack support 
for BGP dynamic routing using the Free Range Routing (FRR) set of 
daemons. Acting as a technical lead for the project, I led one session 
in the TripleO room to discuss the installer components and two sessions 
in the Neutron room to discuss BGP routing with OVN, and BGP EVPN with OVN.

There was some feedback during the Neutron sessions that little has been
done to engage the greater OpenStack/Neutron  community thus far, or to 
utilize the existing RFE process for Neutron. This feedback was correct 
and was received. Initial work was done with a design that didn't 
require changes on Neutron/core OVN to start with. The intention is to 
create Neutron RFEs and to work with others working along similar lines 
now that we have some more clarity as to the direction of further efforts.

There will likely be opportunities to leverage APIs and contribute to
existing work being done with Neutron Dynamic Routing, BGPVPN, and
other work being done to implement BGP EVPN. We would like to
collaborate with Ericsson and others and come up with a solution that
fits us all!

The first steps involved greater changes to the deployment tooling, and
this was proposed and reviewed upstream in the TripleO project:


There are several use cases for using BGP, and in fact there are
separate efforts underway to utilize BGP for the control plane and data

BGP may be used for equal-cost multipath (ECMP) load balancing of
outbound links, and bi-directional forwarding detection (BFD) for
resiliency to ensure that a path provides connectivity. For outbound
connectivity BGP will learn routes from BGP peers. There is separate
work being done to add BFD support for Neutron and Neutron Dynamic
Routing, but this does not provide BFD support for host communication at
the hypervisor layer. Using FRR at the host level provides this BFD support.

BGP may be used for advertising routes to API endpoints. In this model
HAProxy will listen on an IP address and FRR will advertise routes to
that IP to BGP peers. High availability for HAProxy is provided via
other means such as Pacemaker, and FRR will simply advertise the virtual
IP address when it is active on an API controller.

BGP may also be used for routing inbound traffic to provider network IPs
or floating IPs for instance connectivity. The Compute nodes will run
FRR to advertise routes to the local VM IPs or floating IPs hosted on
the node. FRR has a daemon named Zebra that is responsible for
exchanging routes between routing daemons such as BGP and the kernel.
The redistribute connected statement in the FRR configuration will cause
local IP addresses on the host to be advertised via BGP. Floating IP
addresses are attached to a loopback interface in a namespace, so they
will be redistributed using this method. Changes to OVN will be required
to ensure provider network IPs assigned to VMs will be assigned to a
loopback interface in a namespace in a similar fashion.

FRR was selected for integration into TripleO and OVN for several
reasons: using FRR leverages a proven production-grade routing solution
that gives us BGP, BFD (bi-directional forwarding detection), VRFs for
different namespaces for multitenancy, integration with kernel routing,
and potentially other features such as OSPF, RPKI, route
monitoring/mirroring, and more. FRR has a very complete feature set and
is very robust, although there are other BGP speakers available such as
ExaBGP or BIRD, and os-ken, with varying feature sets.

OVN will need to be modified to enable the Compute node to assign VM
provider network IPs to a loopback interface inside a namespace. These
IP address will not be used for sending or receiving traffic, only for
redistributing routes to the IPs to BGP peers. Traffic which is sent to
those IP addresses will be forwarded to the VM using OVS flows on the
hypervisor. An example agent for OVN has been written to demonstrate how
to monitor the southbound OVN DB and create loopback IP addresses when a
VM is started on a Compute node. The OVN changes will be detailed in a
separate OVN spec. Demonstration code is available on Github:

BGP EVPN with multitenancy will require separate VRFs per tenant. This
will allow separate routing tables to be maintained, and allow for
overlapping IP addresses for different Neutron tenant networks. FRR may
have the capability to utilize a single BGP peering session to combine
advertisements for all these VRFs, but there is still work to be done to
prototype this design. This may result in more efficient BGP dynamic
updates, and could potentially make troubleshooting more straightforward.

As suggested in the PTG discussions, we are investigating the BGPVPN 
API. It appears that this API will work well for this use case. 
Hopefully we can make significant progress during the Xena development 
cycle, and we will be able to define what needs to be done in subsequent 
cycles. Any thoughts, suggestions, and contributions are appreciated.

If anyone would like to review the work that we've already published, 
there is a series of blog posts that Luis Tomas Bolivar made related to 
how to use it on OpenStack and how it works:
- OVN-BGP agent introduction:
- How to set ip up on DevStack Environment:
- In-depth traffic flow inspection:

Here are some relevant links to posts written by Luis Tomas Bolivar on 
the ovs-discuss mailing list:

Dan Sneddon         |  Senior Principal Software Engineer
dsneddon at redhat.com |  redhat.com/cloud
dsneddon:irc        |  @dxs:twitter

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