Kubernetes Networking on AWS: A Practical Guide

“If you don’t understand your network, you don’t understand your system.”
— The DevOps Architect

Real-World Problem Introduction

Imagine this: your team is scaling a Kubernetes platform on AWS. Microservices are humming, but as traffic surges, pods mysteriously lose connectivity, inter-service latency spikes, and your cloud bill climbs faster than your Grafana alerts. The culprit? Network complexity—specifically, how Kubernetes overlays mesh with AWS’s own networking.
If you’ve ever struggled to troubleshoot pod IP exhaustion, tangled security groups, or cross-VPC connectivity, you’re not alone. At the heart of these challenges (and their solutions) lies the AWS VPC CNI plugin.

Architecture Context: Why AWS CNI Matters

CNI (Container Network Interface) defines how pods get networked—think of it as the plumbing behind Kubernetes. While CNI plugins abound (Calico, Flannel, Weave), AWS’s own VPC CNI is the go-to for EKS (Elastic Kubernetes Service) clusters, integrating pods directly with native VPC networking.

AWS VPC CNI: How it Differs

Pod IPs Are VPC IPs: Each pod receives an IP address from the VPC subnet, not a separate overlay. This makes network policies, VPC routing, and native AWS tools (like Security Groups, Flow Logs, or Transit Gateway) work seamlessly.
Native Performance: No extra NAT or overlays for intra-VPC communication = lower latency, higher throughput.
AWS IAM Integration: You can apply IAM roles, security groups, and route tables per pod.

Diagram: Pod Networking with AWS VPC CNI

+-------------------------+      +----------------------+
|   Worker Node (EC2)     |      |   AWS VPC Subnet     |
| +---------------------+ |      |                      |
| |  eth0 (Node ENI)    | |<---->|  Subnet CIDR Block   |
| |  +---+---+---+---+  | |      | 10.0.1.0/24          |
| |  | p | p | p | p |  | |      |                      |
| |  +---+---+---+---+  | |      +----------------------+
| +---------------------+ |
+-------------------------+

Each pod receives a VPC IP via ENI attached to the node.

Implementation Details: From Basics to Advanced Tuning

1. How AWS VPC CNI Works

ENI (Elastic Network Interface): Each node starts with a primary ENI (its “main” network adapter) and can attach multiple secondary ENIs, each holding additional IPs.

IP Assignment: Each ENI is assigned a pool of secondary IPs, and the CNI plugin assigns these to pods as they’re scheduled.
Pod Networking: Pods communicate with any VPC resource as if they’re first-class VPC citizens.

Architect’s Note:
The number of pods per node is limited by EC2 instance type (max ENIs × IPs per ENI + primary interface). Always check the AWS docs before scaling!

Check your limites

aws ec2 describe-instance-types \
  --instance-types m5.large \
  --query 'InstanceTypes[*].NetworkInfo'

2. Key Configuration Options

WARM_IP_TARGET: Pre-allocates extra IPs for bursty workloads to reduce pod startup latency.
MINIMUM_IP_TARGET: Ensures a node never falls below a baseline number of free IPs.
ENABLE_POD_ENI: For EKS “pod ENI” mode, allowing pods in “custom networking” to attach their own ENIs (critical for granular IAM/security needs).

Sample ConfigMap:

apiVersion: v1
kind: ConfigMap
metadata:
  name: aws-node
  namespace: kube-system
data:
  WARM_IP_TARGET: "4"
  MINIMUM_IP_TARGET: "2"

3. Custom Networking with Secondary ENIs/Subnets

Secondary Subnets: Assign pods from dedicated subnets, isolating workloads (e.g., frontend pods in public, backend in private).
Security Groups for Pods: Assign SGs per pod (since v1.18), enabling fine-grained traffic control—critical for zero-trust architectures.
IP Prefix Delegation: For high-density clusters, use prefix delegation to assign larger IP blocks, overcoming per-ENI IP limitations.

Enable with:

kubectl set env daemonset aws-node -n kube-system ENABLE_PREFIX_DELEGATION=true

Architect’s Note:
Always validate if your AWS VPC has Prefix Delegation support in your region and CIDR setup before enabling.

4. Monitoring & Troubleshooting

CloudWatch Metrics: The CNI emits metrics like aws_eni_allocated, aws_eni_ip_assigned, and pod_network_setup_latency.
Logging: Enable verbose logging in the aws-node DaemonSet for deep packet-level insight.
Failure Modes: Watch for events like FailedCreatePodSandBox—often a sign of IP exhaustion.

Pitfalls & Optimisations

Common Pitfalls

IP Exhaustion:
- If your subnets are too small, you’ll hit IP limits before CPU or memory.
- Fix: Plan subnet CIDRs based on projected pod scale.
Pod Density Limits:
- EC2 types cap ENIs and IPs. High-density nodes may starve for IPs.
- Fix: Use larger instance types or enable prefix delegation.
Security Gaps:
- Relying solely on Security Groups or Network Policies is risky if not properly combined.
- Fix: Use both; test policies thoroughly.
Cost Surprises:
- Over-provisioned WARM IPs = wasted elastic IPs and higher subnet usage.
- Fix: Tune WARM_IP_TARGET based on actual workloads.

Optimisation Tips

Enable Pod ENI Only When Needed: It’s powerful, but increases ENI usage (cost and quota impact).
Automate Subnet Sizing: Periodically audit subnet usage; consider tools like kube-aws-cni-troubleshooter.
Observability First: Wire up CNI metrics to Grafana/Prometheus for proactive alerting.
Version Management: Keep CNI plugin updated—AWS ships security and performance fixes regularly.

Key Takeaways

AWS VPC CNI gives Kubernetes pods “first-class” AWS networking—but with unique scaling and security considerations.
Planning subnet sizes, understanding ENI/IP limits per instance type, and configuring CNI parameters (like WARM_IP_TARGET) are non-negotiable for production resilience.
Combine Kubernetes Network Policies and AWS Security Groups for Pods for true defense-in-depth.
Proactively monitor CNI metrics to avoid outages and keep pod networking frictionless.

Architect’s Note:
Mastering AWS CNI is the difference between a Kubernetes platform that just works and one that quietly sabotages your scale, security, or cost.

Unlocked: Advanced AWS CNI, Demystified

If you’re running Kubernetes on AWS, don’t treat the VPC CNI plugin as “just another default.” Understand it, tune it, and you’ll unlock resilient, secure, and high-performance networking at scale.

Until next time—keep your clusters healthy and your packets flowing!

Got questions or want deep-dive diagrams/code? Drop them in the comments or reach out—DevOps Unlocked is your platform for mastering the cloud-native world.

A Practical Approach to Kubernetes Networking with AWS CNI