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OVN overlay network on Openshift

This section describes how an OVN overlay network is setup on Openshift 3.10 and later. It explains the various components and how they come together to establish the OVN overlay network. People that are interested in understatnding how the ovn cni plugin is installed will find this useful.

As of OCP-3.10, the Openshift overlay network is managed using daemonsets. The ovs/ovn components are built into a Docker image that has all of the needed packages. Daemonsets are deployed on the nodes in the cluster to provide the network. The daemonsets use a common Docker image when creating the needed pods.


Ovn has a master service that runs on a compute node in the cluster and node controllers that run on all nodes in the cluster. The master service runs ovn north and ovn south daemons that work with the ovs north and south databases. All of the nodes run the ovn controller which accesses the ovn north and south databases.

There are two daemonsets, ovnkube-master.yaml and ovnkube.yaml that mangage the parts of the ovn architecture. The ovnkube-master daemonset runs both master and node services, ovnkube daemonset just runs the node service. The daemonset uses selector labels in the cluster's nodes to specify where the daemonsets run.

Both daemonsets use the same docker image. Configuration is passed from the daemonset to the image using environment variables and mounting directories from the host. The environment variables at set from a configmap that is created during installation. The image includes an control script that is the entrypoint for the container, ovnkube and the openvswitch and ovn rpms.

The ovnkube program (in this repository) provides the interface between Openshift and Ovn. The arguments used when starting it determine its role in the configuration which are master or node.

The ovn cni service is configured and installed after Openshift is installed and running. Openshift comes up with no network support. One of many networking choices, including ovn, is selected, installed, configured and started to complete Openshift installation.

The installer creates a configmap that contains the configuration information. The two daemonsets use the configuration information.

The configmap is mounted in the daemonsets and is used to initialize environment variables that are passed to the Docker image running in the pod.


The daemonsets govern where the components run, what is run, and how they interface with other components. The ovnkube-master.yaml and ovnkube.yaml files bring together the needed pieces and set up the context for running the pods. Here is some of how it is done.

The nodes are chosen based on labels:

    matchLabels: "true"
             or "true"
      nodeSelector: "true"
             or "true"

The daemonset will start a pod on every nodes that has the desired labels.

NOTE: The above labels are openshift labels created when openshift is installed. As this evolves ovn specific labels will be used.

The docker image is configured as follows:

      - name: ovnkube-master
        image: "netdev22:5000/ovn-kube:latest"

The image is in a docker repository that all nodes can access.

NOTE: The above uses a local development docker repository, netdev22:5000

The openshift service account is created during installation and configured here:

      serviceAccountName: ovn

The daemonsets use host networking and provide the cni plugin when they startup. Docker uses the ovn-cni-plugin to access networking. Docker must be able to access the cni plugin that is in the image. To do this the host /opt/cni/bin directory is mounted in the pod and the pod startup script copies the plugin to the host. Since the host operating environment and the pod are different, the installed plugin is just a shim that passes requests to the server that is running in the pod.

      hostNetwork: true
      hostPID: true
        - mountPath: /host/opt/cni/bin
          name: host-opt-cni-bin
        - mountPath: /etc/cni/net.d
          name: host-etc-cni-netd
        - mountPath: /var/lib/cni/networks/ovn-k8s-cni-overlay
          name: host-var-lib-cni-networks-openshift-ovn
      - name: host-opt-cni-bin
          path: /opt/cni/bin
      - name: host-etc-cni-netd
          path: /etc/cni/net.d
      - name: host-var-lib-cni-networks-openshift-ovn
          path: /var/lib/cni/networks/ovn-k8s-cni-overlay

Docker Image

The Dockerfile directs construction of the image. It installs a base OS, rpms, and local scripts into the image. It also sets up directories in the image and copies files into them. In particular it copies in the entrypoint script,

# docker build -t ovn-kube .
# docker tag ovn-kube netdev22:5000/ovn-kube:latest
# docker push netdev22:5000/ovn-kube:latest

The image is taged and pushed to a docker repository. This example uses the local development docker repo, netdev22:5000

NOTE: At present go_controller is built and the resultant files are copied to the docker build directory. In the future, these files will be installed from openvswitch-ovn-kubernetes rpm.


There are three sources of configuration information in the daemonsets:

  1. /etc/origin/node/ files These files are installed by the Openshift install and are in the node daemonset which mounts them on the host. The ovn deamonsets mount them in the pod.
        # Directory which contains the host configuration.
        - mountPath: /etc/origin/node/
          name: host-config
          readOnly: true
        - mountPath: /etc/sysconfig/origin-node
          name: host-sysconfig-node
          readOnly: true
      - name: host-config
          path: /etc/origin/node
      - name: host-sysconfig-node
          path: /etc/sysconfig/origin-node
  1. /etc/openvswitch/ovn_k8s.conf

This file is used by ovn daemons and ovnkube to set needed values. This file is build into the image. It holds configuration constants.

  1. ovn-config configmap

This configmap is created after Openshift is installed and running and before the network is installed. It contains information that is specific to the cluster that is needed for ovn to access the cluster apiserver.

# oc get configmap ovn-config -o yaml
apiVersion: v1
  OvnNorth: tcp://
  OvnSouth: tcp://
kind: ConfigMap

OvnNorth and OvnSouth are used by ovn to access its daemons. They are host IP address of the daemons.

k8s_apiserver allows access to Openshift's api server.

net_cidr and svc_cidr are the configuration configuration cidrs

The configmap keys become environment variable values in the daemonset yaml files.

        - name: OVN_NORTH
              name: ovn-config
              key: OvnNorth
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