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The Container Network Interface (CNI) is a specification and plugin contract that lets a container runtime connect containers to networks and clean up networking resources when containers are removed. In Kubernetes, the runtime invokes a compatible network plugin; CNI is the interface, not one particular networking product.
What CNI does
The CNI specification defines the interface between a container runtime and network plugins. It describes JSON network configuration, how the runtime requests work from a plugin, how plugins can delegate work, and the result and error data exchanged.
In broad terms, the runtime provides configuration and invocation parameters, and the plugin configures connectivity for a container. When the container is deleted, the runtime can invoke cleanup. CNI defines operations for different parts of that lifecycle:
ADDsets up network resources for a container.DELremoves resources allocated for it.CHECKchecks whether the configured network is present and valid.STATUSreports plugin status.VERSIONsupports version negotiation.GCsupports cleanup of stale resources.
The specification page identifies CNI specification version 1.1.0 as current as of October 7, 2026. That is a specification version, not a promise that every runtime, plugin, or library supports every operation in it. Check the compatibility information for the exact versions you plan to run.
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How CNI fits into Kubernetes
Kubernetes expects a cluster network plugin to implement its network model. Kubernetes documentation requires compatibility with CNI v0.4.0 or later and recommends compatibility with v1.0.0. The container runtime—not the Kubernetes scheduler—must be configured to load the plugins it needs.
This division matters when installing or diagnosing a cluster: Kubernetes defines the networking expectations, the runtime invokes the CNI plugin, and the chosen networking implementation configures connectivity. Follow the current instructions for both the runtime and the network provider.
On Kubernetes 1.24 and later, do not follow older guides that tell you to configure kubelet with cni-bin-dir or network-plugin; Kubernetes removed those kubelet parameters in 1.24. The runtime’s current configuration is the relevant place to verify plugin loading.
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Each pod sandbox also needs a loopback interface. Kubernetes documentation describes using the CNI loopback plugin or equivalent runtime behavior. If workloads use Kubernetes hostPort, the network setup must provide port mapping, for example through the official portmap plugin or another implementation with that capability.
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How to choose a CNI implementation
There is no universal best CNI established by the project descriptions below. Select against the cluster’s Kubernetes and runtime versions, operating systems, kernel, managed-cloud support, address plan, security needs, and workload networking requirements. Verify the chosen release’s compatibility matrix and installation guidance.
| Implementation or capability | What it is suited to | Important distinction |
|---|---|---|
| Flannel | Basic Layer 3 connectivity using a host subnet and a chosen forwarding backend, including VXLAN. | Flannel’s daemon does not natively enforce Kubernetes NetworkPolicies; add a policy controller or chain another project if policy enforcement is required. |
| Multus | Pods that need multiple network attachments, including specialized integrations such as SR-IOV, DPDK, OVS-DPDK, or VPP. | It supports multiple attachments; it is not itself a single connectivity design that removes the need to choose and configure the attached networks. |
| OVN-Kubernetes | An overlay networking approach with Open vSwitch-based load balancing and network policy. | Assess its release-specific requirements and operational model against the target cluster. |
Use these as comparison points, not performance rankings. In particular, assess whether the design uses an overlay or underlay, how pod IP addresses are allocated and routed, what policy is enforced by the network plugin itself, and how upgrades, observability, support, and IP capacity will be handled. Flannel, for example, allocates subnet leases per host; its backend choice affects the data plane.
Troubleshoot CNI networking in a useful order
Work from the runtime’s plugin invocation outward to the host network. A pod networking failure can involve configuration, plugin execution, host permissions, address allocation, firewall rules, or the underlying network.
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A free scan shows the junk files, broken settings and background clutter dragging Windows down - then fixes them in one click.Free scan · Windows 10 & 11- Check runtime configuration and logs. Confirm that the runtime has the intended CNI binaries and configuration, then inspect runtime and plugin logs for invocation errors, missing files, or rejected configuration.
- Check address allocation. Confirm that node pod CIDRs are assigned and do not overlap. For Flannel, its troubleshooting guidance recommends inspecting node
podCIDRvalues and avoiding overlapping node subnet ranges. - Check host capabilities and permissions. Verify the plugin has permission to configure routes and interfaces and that required kernel networking support is available. Flannel documents permission-related route, VXLAN, and masquerading failures, and notes a
br_netfilterrequirement in its project documentation. - Check backend-specific firewall rules. Confirm that the traffic required by the configured backend is allowed between nodes. Flannel documents UDP 8285 for its UDP backend and UDP 8472 for VXLAN; treat these as backend-specific documented values, not universal CNI ports, and verify the active configuration before changing firewall rules.
- Check MTU across the path. Compare the physical or underlay interface, any encapsulation path, and the pod virtual Ethernet interface. Tunnels reduce the space available for pod packets, and a mismatch can cause connectivity problems that affect only larger packets.
- Check cluster and host health. If new hosts are slow to become reachable, inspect control-plane and backing datastore or API health, as well as node CPU and memory availability.
What CNI does not tell you
CNI standardizes the runtime-to-plugin contract; it does not make all plugins interchangeable in behavior or operating model. The specification alone does not establish which implementation will be fastest, easiest to operate, or supported by a particular managed Kubernetes service. Those questions depend on the implementation release and the target environment, so use the provider’s version-specific compatibility and deployment documentation.
