Container Network Interface (CNI) in Kubernetes - Step-by-Step Execution

Process Flow - Container Network Interface (CNI)

Pod requests network

↓

Kubelet calls CNI plugin

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CNI plugin configures network

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Assign IP & setup routes

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Pod network ready

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Pod communicates over network

Shows how Kubernetes uses CNI plugins to set up networking for pods step-by-step.

Execution Sample

Kubernetes

kubectl apply -f pod.yaml
# Pod starts
# Kubelet calls CNI plugin
# CNI assigns IP
# Pod network ready

This sequence shows how a pod's network is set up using CNI when the pod starts.

Process Table

Step	Action	Component	Result	Notes
1	Pod creation requested	User/Kubernetes API	Pod object created	Pod spec includes network request
2	Kubelet detects pod start	Kubelet	Calls CNI plugin	Triggers network setup
3	CNI plugin invoked	CNI plugin	Reads config and allocates IP	Allocates IP from network pool
4	Network interfaces configured	CNI plugin	Network namespace setup	Routes and interfaces added
5	Pod network ready	Kubelet	Pod can send/receive packets	Pod joins cluster network
6	Pod communicates	Pod	Network traffic flows	Pod uses assigned IP
7	Pod deleted	User/Kubernetes API	Kubelet calls CNI DEL	Network resources cleaned up
8	CNI cleans network	CNI plugin	IP released and interfaces removed	Prepares for next pod
9	End	System	Pod network lifecycle complete	Network resources freed

💡 Pod deleted and network cleaned up, ending the network lifecycle

Status Tracker

Variable	Start	After Step 3	After Step 4	After Step 5	Final
Pod IP	None	10.244.1.5	10.244.1.5	10.244.1.5	None
Network Interfaces	None	eth0 created	eth0 configured	eth0 active	Removed
Routing Table	Empty	Routes added	Routes active	Routes active	Routes removed

Key Moments - 3 Insights

Why does the pod have no IP before the CNI plugin runs?

Before step 3 in the execution_table, the pod network is not set up, so no IP is assigned yet.

What happens if the CNI plugin fails to assign an IP?

The pod network setup fails at step 3, so the pod cannot communicate and will not become ready.

Why is the network cleaned up after pod deletion?

At steps 7 and 8, the CNI plugin removes interfaces and releases IPs to avoid resource leaks for future pods.

Visual Quiz - 3 Questions

Look at the execution_table, what IP is assigned to the pod after step 3?

A 10.244.1.5

B None

C 192.168.0.1

D 127.0.0.1

💡 Hint

Check the 'Pod IP' variable in variable_tracker after step 3

At which step does the pod network become ready for communication?

A Step 4

B Step 5

C Step 2

D Step 7

💡 Hint

Look for 'Pod network ready' in the execution_table

If the pod is deleted, what happens to the network interfaces?

A They remain active

B They are duplicated

C They are removed

D They are renamed

💡 Hint

See steps 7 and 8 in execution_table and variable_tracker final state

Concept Snapshot

Container Network Interface (CNI) in Kubernetes:
- Kubelet calls CNI plugin to set up pod network
- CNI plugin assigns IP and configures interfaces
- Pod network ready after setup
- On pod deletion, CNI cleans network resources
- Enables pod communication inside cluster

Full Transcript

Container Network Interface (CNI) is how Kubernetes sets up networking for pods. When a pod starts, the kubelet calls the CNI plugin. The plugin assigns an IP address and configures network interfaces and routes inside the pod's network namespace. Once done, the pod can communicate over the cluster network. When the pod is deleted, the kubelet calls the CNI plugin to clean up network resources like IP addresses and interfaces. This process ensures pods have isolated, functional networking and resources are reused efficiently.