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Chapter 1. Understanding multiple networks

By default, OVN-Kubernetes serves as the Container Network Interface (CNI) of an OpenShift Container Platform cluster. With OVN-Kubernetes as the default CNI of a cluster, OpenShift Container Platform administrators or users can leverage user-defined networks (UDNs) or NetworkAttachmentDefinition (NADs) to create one, or multiple, default networks that handle all ordinary network traffic of the cluster. Both user-defined networks and Network Attachment Definitions can serve as the following network types:

  • Primary networks: Act as the primary network for the pod. By default, all traffic passes through the primary network unless a pod route is configured to send traffic through other networks.
  • Secondary networks: Act as secondary, non-default networks for a pod. Secondary networks provide separate interfaces dedicated to specific traffic types or purposes. Only pod traffic that is explicitly configured to use a secondary network is routed through its interface.

However, during cluster installation, OpenShift Container Platform administrators can configure alternative default secondary pod networks by leveraging the Multus CNI plugin. With Multus, multiple CNI plugins such as ipvlan, macvlan, or Network Attachment Definitions can be used together to serve as secondary networks for pods.

Note

User-defined networks are only available when OVN-Kubernetes is used as the CNI. They are not supported for use with other CNIs.

You can define an secondary network based on the available CNI plugins and attach one or more of these networks to your pods. You can define more than one secondary network for your cluster depending on your needs. This gives you flexibility when you configure pods that deliver network functionality, such as switching or routing.

For a complete list of supported CNI plugins, see "Secondary networks in OpenShift Container Platform".

For information about user-defined networks, see About user-defined networks (UDNs).

For information about Network Attachment Definitions, see Creating primary networks using a NetworkAttachmentDefinition.

1.1. Usage scenarios for a secondary network
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You can use a secondary network in situations where network isolation is needed, including data plane and control plane separation. Isolating network traffic is useful for the following performance and security reasons:

  1. Performance

    Traffic management: You can send traffic on two different planes to manage how much traffic is along each plane.

  2. Security

    Network isolation: You can send sensitive traffic onto a network plane that is managed specifically for security considerations, and you can separate private data that must not be shared between tenants or customers.

All of the pods in the cluster still use the cluster-wide default network to maintain connectivity across the cluster. Every pod has an eth0 interface that is attached to the cluster-wide pod network. You can view the interfaces for a pod by using the oc exec -it <pod_name> -- ip a command. If you add secondary network interfaces that use Multus CNI, they are named net1, net2, …​, netN.

To attach secondary network interfaces to a pod, you must create configurations that define how the interfaces are attached. You specify each interface by using either a UserDefinedNetwork custom resource (CR) or a NetworkAttachmentDefinition CR. A CNI configuration inside each of these CRs defines how that interface is created.

For more information about creating a UserDefinedNetwork CR, see About user-defined networks.

For more information about creating a NetworkAttachmentDefinition CR, see Creating primary networks using a NetworkAttachmentDefinition.

1.2. Secondary networks in OpenShift Container Platform
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OpenShift Container Platform provides the following CNI plugins for creating secondary networks in your cluster:

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