Chapter 1. Understanding multiple networks

By default, OVN-Kubernetes serves as the Container Network Interface (CNI) of an OpenShift Container Platform cluster. This network interface is what administrators use to create default networks.

Both user-defined networks and Network Attachment Definitions can serve as the following network types:

The following diagram shows a cluster that has an existing default network infrastructure that uses a physical network interface, eth0, to connect to two namespaces. Pods or virtual machines (VMs) in one namespace run in isolation from pods or VMs in the other namespace. You can create only one primary network. However, you can create multiple secondary networks for each namespace.

Figure 1.1. Diagram showing namespaces with multiple secondary UDNs

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

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 more information, see the the links in the Additional resources:

You can use user defined networks and network attachment definitions to define and configure customized networks for your needs.

By creating UserDefinedNetwork and NetworkAttachmentDefinition custom resources (CRs), cluster administrators can complete the following tasks:

By creating a ClusterUserDefinedNetwork CR, administrators can create and define secondary networks that span multiple namespaces at the cluster level.

User-defined networks and network attachment definitions can serve as both the primary and secondary network interface, and each support layer2 and layer3 topologies.

The following section highlights the supported features of the UserDefinedNetwork and NetworkAttachmentDefinition CRs when used as either the primary or secondary network. A separate table for the ClusterUserDefinedNetwork CR is also included.

where:

The Localnet topology is unavailable for use with the UserDefinedNetwork CR. It is only supported on secondary networks for NetworkAttachmentDefinition CRs.

where: