Appendix O: Open Virtual Network (OVN)


As of the 19.10 OpenStack Charms release, with OpenStack Train or later, support for integration with Open Virtual Network (OVN) is available. As of the 20.05 OpenStack Charms release OVN is the preferred default for our OpenStack Base bundle reference implementation.


There are feature gaps from ML2/OVS and deploying legacy ML2/OVS with the OpenStack Charms is still available.

OVN charms:

  • neutron-api-plugin-ovn

  • ovn-central

  • ovn-chassis

  • ovn-dedicated-chassis


OVN makes use of Public Key Infrastructure (PKI) to authenticate and authorize control plane communication. The charm requires a Certificate Authority to be present in the model as represented by the certificates relation.

Follow the instructions for deployment and configuration of Vault in the Vault and Certificate Lifecycle Management appendices.

OVN can then be deployed:

juju config neutron-api manage-neutron-plugin-legacy-mode=false

juju deploy neutron-api-plugin-ovn
juju deploy ovn-central -n 3 --config source=cloud:bionic-ussuri
juju deploy ovn-chassis

juju add-relation neutron-api-plugin-ovn:certificates vault:certificates
juju add-relation neutron-api-plugin-ovn:neutron-plugin \
juju add-relation neutron-api-plugin-ovn:ovsdb-cms ovn-central:ovsdb-cms
juju add-relation ovn-central:certificates vault:certificates
juju add-relation ovn-chassis:ovsdb ovn-central:ovsdb
juju add-relation ovn-chassis:certificates vault:certificates
juju add-relation ovn-chassis:nova-compute nova-compute:neutron-plugin

The OVN components used for the data plane is deployed by the ovn-chassis subordinate charm. A subordinate charm is deployed together with a principle charm, nova-compute in the example above.

If you require a dedicated software gateway you may deploy the data plane components as a principle charm through the use of the ovn-dedicated-chassis charm.


For a concrete example take a look at the OpenStack Base bundle.

High availability

OVN is HA by design; take a look at the OVN section of the OpenStack high availability appendix.


OVN integrates with OpenStack through the OVN ML2 driver. On OpenStack Ussuri and onwards the OVN ML2 driver is maintained as an in-tree driver in Neutron. On OpenStack Train it is maintained separately as per the networking-ovn plugin.

General Neutron configuration is still done through the neutron-api charm, and the subset of configuration specific to OVN is done through the neutron-api-plugin-ovn charm.

Hardware offloading support

It is possible to configure chassis to prepare network interface cards (NICs) for use with hardware offloading and make them available to OpenStack.


Support for hardware offload in conjunction with OVN is an experimental feature. OVN programs flow tables in a different way than legacy ML2+OVS and this has had less exposure to validation in NIC firmware and driver support.

To use the feature you need to use supported network interface card (NIC) hardware. We have done feature validation using the Mellanox ConnectX-5 NICs.

Please refer to the NIC hardware offload appendix for more background on the feature.

Hardware offload support makes use of SR-IOV as an underlying mechanism to accelerate the data path between a virtual machine instance and the NIC hardware. But as opposed to traditional SR-IOV support the accelerated ports can be connected to the Open vSwitch integration bridge which allows instances to take part in regular tenant networks. The NIC also supports hardware offloading of tunnel encapsulation and decapsulation.

With OVN the Layer3 routing features are implemented as flow rules in Open vSwitch. This in turn may allow Layer 3 routing to also be offloaded to NICs with appropriate driver and firmware support.


Please refer to the SR-IOV for networking support section and the NIC hardware offload appendix for information on hardware and kernel configuration.

Charm configuration

The below example bundle excerpt will enable hardware offloading for an OVN deployment.

    charm: cs:ovn-chassis
      enable-hardware-offload: true
      sriov-numvfs:  "enp3s0f0:64 enp3s0f1:0"
    charm: cs:neutron-api
      enable-hardware-offload: true
    charm: cs:nova-compute
      pci-passthrough-whitelist: '{"address": "*:03:*", "physical_network": null}'

Boot an instance

Now we can tell OpenStack to boot an instance and attach it to an hardware offloaded port. This must be done in two stages, first we create a port with vnic-type ‘direct’ and binding-profile with ‘switchdev’ capabilities. Then we create an instance connected to the newly created port:

openstack port create --network my-network --vnic-type direct \
    --binding-profile '{"capabilities": ["switchdev"]}' direct_port1
openstack server create --flavor my-flavor --key-name my-key \
    --nic port-id=direct_port1 my-instance

Validate that traffic is offloaded

The traffic control monitor command can be used to observe updates to filters which is one of the mechanisms used to program the NIC switch hardware. Look for the ‘in_hw’ and ‘not_in_hw’ labels.

sudo tc monitor
replaced filter dev eth62 ingress protocol ip pref 3 flower chain 0 handle 0x9
  dst_mac fa:16:3e:b2:20:82
  src_mac fa:16:3e:b9:db:c8
  eth_type ipv4
  ip_proto tcp
  ip_tos 67deeb90
  tcp_flags 22
  ip_flags nofrag
    action order 1: tunnel_key set
    key_id 4
    dst_port 6081
    csum pipe
    index 15 ref 1 bind 1

    action order 2: mirred (Egress Redirect to device genev_sys_6081) stolen
    index 18 ref 1 bind 1
    cookie d4885b4d38419f7fd7ae77a11bc78b0b

Open vSwitch has a rich set of tools to monitor traffic flows and you can use the data path control tools to monitor offloaded flows.

sudo ovs-appctl dpctl/dump-flows type=offloaded
tunnel(tun_id=0x4,src=,dst=,tp_dst=6081,geneve({class=0x102,type=0x80,len=4,0x20007/0x7fffffff}),flags(+key)),recirc_id(0),in_port(2),eth(src=fa:16:3e:f8:52:5c,dst=00:00:00:00:00:00/01:00:00:00:00:00),eth_type(0x0800),ipv4(proto=6,frag=no),tcp_flags(psh|ack), packets:2, bytes:204, used:5.710s, actions:7
tunnel(tun_id=0x4,src=,dst=,tp_dst=6081,geneve({class=0x102,type=0x80,len=4,0x20007/0x7fffffff}),flags(+key)),recirc_id(0),in_port(2),eth(src=fa:16:3e:f8:52:5c,dst=00:00:00:00:00:00/01:00:00:00:00:00),eth_type(0x0800),ipv4(proto=6,frag=no),tcp_flags(ack), packets:3, bytes:230, used:5.710s, actions:7
tunnel(tun_id=0x4,src=,dst=,tp_dst=6081,geneve({class=0x102,type=0x80,len=4,0x60007/0x7fffffff}),flags(+key)),recirc_id(0),in_port(2),eth(src=fa:16:3e:b2:20:82,dst=00:00:00:00:00:00/01:00:00:00:00:00),eth_type(0x0800),ipv4(proto=6,frag=no),tcp_flags(syn|ack), packets:0, bytes:0, used:6.740s, actions:7
tunnel(tun_id=0x4,src=,dst=,tp_dst=6081,geneve({class=0x102,type=0x80,len=4,0x60007/0x7fffffff}),flags(+key)),recirc_id(0),in_port(2),eth(src=fa:16:3e:b2:20:82,dst=00:00:00:00:00:00/01:00:00:00:00:00),eth_type(0x0800),ipv4(proto=6,frag=no),tcp_flags(ack), packets:180737, bytes:9400154, used:0.000s, actions:7
recirc_id(0),in_port(6),eth(src=26:8a:07:82:a7:2f,dst=01:80:c2:00:00:0e),eth_type(0x88cc), packets:5, bytes:990, used:14.340s, actions:drop
recirc_id(0),in_port(7),eth(src=fa:16:3e:b9:db:c8,dst=fa:16:3e:b2:20:82),eth_type(0x0800),ipv4(dst=,proto=6,tos=0/0x3,frag=no),tcp_flags(syn), packets:0, bytes:0, used:6.910s, actions:set(tunnel(tun_id=0x4,dst=,ttl=64,tp_dst=6081,key6(bad key length 1, expected 0)(01)geneve({class=0x102,type=0x80,len=4,0x70006}),flags(key))),2
recirc_id(0),in_port(7),eth(src=fa:16:3e:b9:db:c8,dst=fa:16:3e:b2:20:82),eth_type(0x0800),ipv4(dst=,proto=6,tos=0/0x3,frag=no),tcp_flags(ack), packets:935904, bytes:7504070178, used:0.590s, actions:set(tunnel(tun_id=0x4,dst=,ttl=64,tp_dst=6081,key6(bad key length 1, expected 0)(01)geneve({class=0x102,type=0x80,len=4,0x70006}),flags(key))),2
recirc_id(0),in_port(7),eth(src=fa:16:3e:b9:db:c8,dst=fa:16:3e:b2:20:82),eth_type(0x0800),ipv4(dst=,proto=6,tos=0/0x3,frag=no),tcp_flags(psh|ack), packets:3873, bytes:31053714, used:0.590s, actions:set(tunnel(tun_id=0x4,dst=,ttl=64,tp_dst=6081,key6(bad key length 1, expected 0)(01)geneve({class=0x102,type=0x80,len=4,0x70006}),flags(key))),2

SR-IOV for networking support

Single root I/O virtualization (SR-IOV) enables splitting a single physical network port into multiple virtual network ports known as virtual functions (VFs). The division is done at the PCI level which allows attaching the VF directly to a virtual machine instance, bypassing the networking stack of the hypervisor hosting the instance.

The main use case for this feature is to support applications with high bandwidth requirements. For such applications the normal plumbing through the userspace virtio driver in QEMU will consume too much resources from the host.

It is possible to configure chassis to prepare network interface cards (NICs) for use with SR-IOV and make them available to OpenStack.


To use the feature you need to use a NIC with support for SR-IOV.

Machines need to be pre-configured with appropriate kernel command-line parameters. The charm does not handle this facet of configuration and it is expected that the user configure this either manually or through the bare metal provisioning layer (for example MAAS). Example:

intel_iommu=on iommu=pt probe_vf=0

Charm configuration

Enable SR-IOV, map physical network name ‘physnet2’ to the physical port named ‘enp3s0f0’ and create 4 virtual functions on it:

juju config ovn-chassis enable-sriov=true
juju config ovn-chassis sriov-device-mappings=physnet2:enp3s0f0
juju config ovn-chassis sriov-numvfs=enp3s0f0:4

After enabling the virtual functions you should take note of the vendor_id and product_id of the virtual functions:

juju run --application ovn-chassis 'lspci -nn | grep "Virtual Function"'
03:10.0 Ethernet controller [0200]: Intel Corporation 82599 Ethernet Controller Virtual Function [8086:10ed] (rev 01)
03:10.2 Ethernet controller [0200]: Intel Corporation 82599 Ethernet Controller Virtual Function [8086:10ed] (rev 01)
03:10.4 Ethernet controller [0200]: Intel Corporation 82599 Ethernet Controller Virtual Function [8086:10ed] (rev 01)
03:10.6 Ethernet controller [0200]: Intel Corporation 82599 Ethernet Controller Virtual Function [8086:10ed] (rev 01)

In the above example vendor_id is ‘8086’ and product_id is ‘10ed’.

Add mapping between physical network name, physical port and Open vSwitch bridge:

juju config ovn-chassis ovn-bridge-mappings=physnet2:br-ex
juju config ovn-chassis bridge-interface-mappings br-ex:a0:36:9f:dd:37:a8


The above configuration allows OVN to configure an ‘external’ port on one of the chassis for providing DHCP and metadata to instances connected directly to the network through SR-IOV.

For OpenStack to make use of the VFs the neutron-sriov-agent needs to talk to RabbitMQ:

juju add-relation ovn-chassis:amqp rabbitmq-server:amqp

OpenStack Nova also needs to know which PCI devices it is allowed to pass through to instances:

juju config nova-compute pci-passthrough-whitelist='{"vendor_id":"8086", "product_id":"10ed", "physical_network":"physnet2"}'

Boot an instance

Now we can tell OpenStack to boot an instance and attach it to an SR-IOV port. This must be done in two stages, first we create a port with vnic-type ‘direct’ and then we create an instance connected to the newly created port:

openstack port create --network my-network --vnic-type direct my-port
openstack server create --flavor my-flavor --key-name my-key \
   --nic port-id=my-port my-instance

DPDK support

It is possible to configure chassis to use experimental DPDK userspace network acceleration.


Currently instances are required to be attached to a external network (also known as provider network) for connectivity. OVN supports distributed DHCP for provider networks. For OpenStack workloads use of Nova config drive is required to provide metadata to instances.


To use the feature you need to use a supported CPU architecture and network interface card (NIC) hardware. Please consult the DPDK supported hardware page.

Machines need to be pre-configured with appropriate kernel command-line parameters. The charm does not handle this facet of configuration and it is expected that the user configure this either manually or through the bare metal provisioning layer (for example MAAS).


default_hugepagesz=1G hugepagesz=1G hugepages=64 intel_iommu=on iommu=pt

For the communication between the host userspace networking stack and the guest virtual NIC driver to work the instances need to be configured to use hugepages. For OpenStack this can be accomplished by Customizing instance huge pages allocations.


openstack flavor set m1.large --property hw:mem_page_size=large

By default, the charm will configure Open vSwitch/DPDK to consume one processor core + 1G of RAM from each NUMA node on the unit being deployed. This can be tuned using the dpdk-socket-memory and dpdk-socket-cores configuration options. The userspace kernel driver can be configured using the dpdk-driver configuration option. See config.yaml for more details.


Changing dpdk related configuration options will trigger a restart of Open vSwitch, and subsequently interrupt instance connectivity.

DPDK bonding

Once Network interface cards are bound to DPDK they will be invisible to the standard Linux kernel network stack and subsequently it is not possible to use standard system tools to configure bonding.

For DPDK interfaces the charm supports configuring bonding in Open vSwitch. This is accomplished through the dpdk-bond-mappings and dpdk-bond-config configuration options. Example:

    enable-dpdk: True
    bridge-interface-mappings: br-ex:dpdk-bond0
    dpdk-bond-mappings: "dpdk-bond0:00:53:00:00:00:42 dpdk-bond0:00:53:00:00:00:51"
    dpdk-bond-config: ":balance-slb:off:fast"

In this example, the network interface cards associated with the two MAC addresses provided will be used to build a bond identified by a port named ‘dpdk-bond0’ which will be attached to the ‘br-ex’ bridge.

Internal DNS resolution

OVN supports Neutron internal DNS resolution. To configure this:

juju config neutron-api enable-ml2-dns=true
juju config neutron-api dns-domain=openstack.example.
juju config neutron-api-plugin-api dns-servers=""


The value for the dns-domain configuration option must not be set to ‘openstack.local.’ as that will effectively disable the feature.

It is also important to end the string with a ‘.’ (dot).

When you set enable-ml2-dns to ‘true’ and set a value for dns-domain, Neutron will add details such as instance name and DNS domain name to each individual Neutron port associated with instances. The OVN ML2 driver will populate the DNS table of the Northbound and Southbound databases:

# ovn-sbctl list DNS
_uuid               : 2e149fa8-d27f-4106-99f5-a08f60c443bf
datapaths           : [b25ed99a-89f1-49cc-be51-d215aa6fb073]
external_ids        : {dns_id="4c79807e-0755-4d17-b4bc-eb57b93bf78d"}

records             : {"c-1"="", "c-1.openstack.example"=""}

On the chassis, OVN creates flow rules to redirect UDP port 53 packets (DNS) to the local ovn-controller process:

cookie=0xdeaffed, duration=77.575s, table=22, n_packets=0, n_bytes=0, idle_age=77, priority=100,udp6,metadata=0x2,tp_dst=53 actions=controller(,pause),resubmit(,23)
cookie=0xdeaffed, duration=77.570s, table=22, n_packets=0, n_bytes=0, idle_age=77, priority=100,udp,metadata=0x2,tp_dst=53 actions=controller(,pause),resubmit(,23)

The local ovn-controller process then decides if it should respond to the DNS query directly or if it needs to be forwarded to the real DNS server.

External connectivity

Interface and network to bridge mapping is done through the ovn-chassis charm.

OVN provides a more flexible way of configuring external Layer3 networking than the legacy ML2+DVR configuration as OVN does not require every node (Chassis in OVN terminology) in a deployment to have direct external connectivity. This plays nicely with Layer3-only datacenter fabrics (RFC 7938).

East/West traffic is distributed by default. North/South traffic is highly available by default. Liveness detection is done using the Bidirectional Forwarding Detection (BFD) protocol.

Networks for use with external Layer3 connectivity should have mappings on chassis located in the vicinity of the datacenter border gateways. Having two or more chassis with mappings for a Layer3 network will have OVN automatically configure highly available routers with liveness detection provided by the Bidirectional Forwarding Detection (BFD) protocol.

Chassis without direct external mapping to a external Layer3 network will forward traffic through a tunnel to one of the chassis acting as a gateway for that network.


It is not necessary nor recommended to add mapping for external Layer3 networks to all chassis. Doing so will create a scaling problem at the physical network layer that needs to be resolved with globally shared Layer2 (does not scale) or tunneling at the top-of-rack switch layer (adds complexity) and is generally not a recommended configuration.

Example configuration:

juju config neutron-api flat-network-providers=physnet1
juju config ovn-chassis ovn-bridge-mappings=physnet1:br-provider
juju config ovn-chassis \
    bridge-interface-mappings='br-provider:00:00:5e:00:00:42 \
openstack network create --external --share --provider-network-type flat \
                         --provider-physical-network physnet1 ext-net
openstack subnet create --network ext-net \
                        --subnet-range \
                        --no-dhcp --gateway \

Networks for use with external Layer2 connectivity should have mappings present on all chassis with potential to host the consuming payload.


Create networks, routers and subnets through the OpenStack API or CLI as you normally would.

The OVN ML2 driver will translate the OpenStack network constructs into high level logical rules in the OVN Northbound database.

The ovn-northd daemon in turn translates this into data in the Southbound database.

The local ovn-controller daemon on each chassis consumes these rules and programs flows in the local Open vSwitch database.

Information queries

The OVN databases are configured to use the Clustered Database Service Model. In this configuration only the leader processes transactions and the administrative client tools are configured to require a connection to the leader to operate.

The leader of the Northbound and Southbound databases does not have to coincide with the charm leader, so before querying databases you must consult the output of juju status to check which unit is the leader of the database you want to query. Example:

juju status ovn-central
Unit            Workload  Agent  Machine  Public address  Ports              Message
ovn-central/0*  active    idle   0/lxd/5   6641/tcp,6642/tcp  Unit is ready (leader: ovnnb_db)
ovn-central/1   active    idle   1/lxd/4   6641/tcp,6642/tcp  Unit is ready (northd: active)
ovn-central/2   active    idle   2/lxd/2   6641/tcp,6642/tcp  Unit is ready (leader: ovnsb_db)

In the above example ‘ovn-central/0’ is the leader for the Northbound DB, ‘ovn-central/1’ has the active ovn-northd daemon and ‘ovn-central/2’ is the leader for the Southbound DB.

OVSDB Cluster status

The cluster status as conveyed through juju status is updated each time a hook is run, in some circumstances it may be necessary to get an immediate view of the current cluster status.

To get an immediate view of the database clusters:

juju run --application ovn-central 'ovn-appctl -t \
    /var/run/ovn/ovnnb_db.ctl cluster/status OVN_Northbound'
juju run --application ovn-central 'ovn-appctl -t \
    /var/run/ovn/ovnsb_db.ctl cluster/status OVN_Southbound'

Querying DBs

To query the individual databases:

juju run --unit ovn-central/0 'ovn-nbctl show'
juju run --unit ovn-central/2 'ovn-sbctl show'
juju run --unit ovn-central/2 'ovn-sbctl lflow-list'

As an alternative you may provide the administrative client tools with command-line arguments for path to certificates and IP address of servers so that you can run the client from anywhere:

ovn-nbctl \
   -p /etc/ovn/key_host \
   -C /etc/ovn/ovn-central.crt \
   -c /etc/ovn/cert_host \
   --db ssl:,ssl:,ssl: \

Note that for remote administrative write access to the Southbound DB you must use port number ‘16642’. This is due to OVN RBAC being enabled on the standard ‘6642’ port:

ovn-sbctl \
   -p /etc/ovn/key_host \
   -C /etc/ovn/ovn-central.crt \
   -c /etc/ovn/cert_host \
   --db ssl:,ssl:,ssl: \

Data plane flow tracing

SSH into one of the chassis units to get access to various diagnostic tools:

juju ssh ovn-chassis/0

sudo ovs-vsctl show

sudo ovs-ofctl -O OpenFlow13 dump-flows br-int

sudo ovs-appctl -t ovs-vswitchd \
   ofproto/trace br-provider \

sudo ovn-trace \
   -p /etc/ovn/key_host \
   -C /etc/ovn/ovn-chassis.crt \
   -c /etc/ovn/cert_host \
   --db ssl:,ssl:,ssl: \
   --ovs ext-net 'inport=="provnet-dde76bc9-0620-44f7-b99a-99cfc66e1095" && \
   eth.src==30:e1:71:5c:7a:b5 && \
   eth.dst==fa:16:3e:f7:15:73 && \
   ip4.src== && \
   ip4.dst== && \
   icmp4.type==8 && \
   ip.ttl == 64'


OVN makes use of OpenFlow 1.3 or newer and as such the charm configures bridges to use these protocols. To be able to successfully use the ovs-ofctl command you must specify the OpenFlow version as shown in the example above.

You may issue the ovs-vsctl list bridge command to show what protocols are enabled on the bridges.