CKA Troubleshooting Practice Question
You have a multi-node cluster. One node shows 'NotReady'. You run 'journalctl -u kubelet' on that node and see 'network plugin is not ready'. What is the most likely cause?
⚠ Common exam trap
Candidates often confuse 'network plugin is not ready' with a general network connectivity issue or a kubelet failure, but the error specifically points to the CNI plugin not being operational on that node, not the kubelet itself or the container runtime.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The CNI plugin pod (e.g., Calico) is not running on that node
The 'network plugin is not ready' error from kubelet indicates that the kubelet is waiting for a CNI (Container Network Interface) plugin to configure the pod network. If the CNI plugin pod (e.g., Calico, Flannel, Weave) is not running on that specific node, the kubelet cannot set up the network for pods, causing the node to remain in 'NotReady' state. This is the most direct cause because the kubelet relies on the CNI plugin to report readiness before marking the node as Ready.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
The CNI plugin pod (e.g., Calico) is not running on that node
Why this is correct
The kubelet agent on each node continuously monitors the health and operational status of the CNI plugin. If the CNI plugin's pod (e.g., Calico, Flannel) crashes, fails its readiness probes, or is not running on a specific node, the kubelet cannot properly configure network interfaces for pods or establish essential network routes. Consequently, the kubelet reports the node's status as `NotReady` with the specific `NetworkPluginNotReady` condition, indicating a fundamental failure in the node's networking capabilities.
- ✗
The container runtime (e.g., containerd) is down
Why it's wrong here
If the container runtime (e.g., containerd, CRI-O) were down on a node, the kubelet would be unable to start, stop, or manage any containers. This would typically manifest as `ContainerRuntimeNotReady` or `KubeletNotReady` conditions, and pods would fail to launch or enter a `CrashLoopBackOff` state due to image pull failures or execution errors. The specific "network plugin not ready" message is distinct, indicating a problem with the CNI layer rather than the underlying container execution environment.
- ✗
The kubelet service is not running
Why it's wrong here
A non-running kubelet service would prevent the node from registering with the Kubernetes API server or updating its status entirely. The API server would eventually mark the node as `NotReady` due to a lack of heartbeats, but without any specific reason like "network plugin not ready" being reported by the node itself. Furthermore, `journalctl` on the node would clearly show the kubelet service as failed or inactive, which is a more general and severe problem than a specific CNI issue.
- ✗
The node's IP address has changed
Why it's wrong here
While a node's IP address changing could certainly lead to network connectivity problems, such as pods losing communication or services becoming unreachable, it does not directly trigger the `NetworkPluginNotReady` condition. The CNI plugin itself is designed to handle network configuration based on the node's current network setup, and an IP change would likely cause broader network disruption or DNS issues. The `NetworkPluginNotReady` status specifically indicates a failure in the CNI plugin's operational health as reported by the kubelet.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This CKA practice question is part of Courseiva's free CNCF certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the CKA exam.