Networking is a foundational pillar of any Kubernetes cluster, yet it remains one of the most complex and nuanced aspects to master. Choosing the right networking model impacts cluster scalability, security, and performance. This post dives deep into three leading Kubernetes networking solutions — Flannel, Calico, and Cilium — dissecting their architectures, features, and use cases to help intermediate and advanced users make informed decisions.

Kubernetes abstracts networking by requiring that:

  • Every Pod gets its own unique IP address.
  • Pods can communicate with each other across nodes without NAT.
  • Network policies control traffic flow and security.

These goals are implemented by Container Network Interface (CNI) plugins — modular networking components that enable container networking in Kubernetes.

Flannel: Simple Overlay Networking

Flannel is one of the oldest and most straightforward Kubernetes CNIs designed to create a simple overlay network.

  • Uses VXLAN or UDP tunneling to encapsulate Pod traffic.
  • Creates a flat Layer 3 network where every Pod can reach every other Pod.
  • Provides minimal or no native network policy enforcement.
  • Focuses on ease of setup and broad compatibility.

Architecture Highlights:

  • Flannel runs a daemonset on each node.
  • Each node gets assigned a subnet for Pods.
  • Traffic between nodes is encapsulated inside VXLAN packets.
  • Limited performance overhead but added latency due to encapsulation.

Use cases: Best suited for small to medium clusters where simplicity and quick setup matter more than advanced network policies or performance tuning.

Calico: High-Performance Networking and Network Policies

Calico brings advanced networking and security to Kubernetes by combining pure Layer 3 routing with powerful network policy enforcement.

  • Uses BGP routing to advertise Pod IPs across nodes.
  • Supports both overlay (VXLAN) and native routing modes.
  • Implements Kubernetes NetworkPolicy with fine-grained rules.
  • Provides IP-in-IP encapsulation fallback for non-routable environments.

Architecture Highlights:

  • Calico runs an agent called calico-node on each cluster node.
  • Uses BGP to distribute routing info, eliminating encapsulation overhead when possible.
  • Integrates with Kubernetes RBAC and policy APIs.
  • Supports eBPF data plane acceleration in newer versions.

Use cases: Ideal for production-grade environments needing scalable, high-performance networking combined with strict security controls.

Cilium: eBPF-Powered Networking and Security

Cilium leverages the Linux kernel’s eBPF (extended Berkeley Packet Filter) technology to provide high-performance networking with deep visibility and security capabilities.

  • Implements Layer 3/L4 networking with eBPF-based datapath.
  • Supports Kubernetes NetworkPolicies plus extended Layer 7 policies.
  • Provides transparent encryption and load balancing.
  • Offers built-in observability tools (Hubble) for real-time monitoring.

Architecture Highlights:

  • Runs a daemonset with an agent on every node.
  • Injects eBPF programs directly into the Linux kernel for packet processing.
  • Avoids traditional overlay networks in favor of native Linux networking primitives.
  • Enables context-aware security policies at application protocol level.

Use cases: Best suited for security-conscious environments, complex multi-tenant clusters, and those needing advanced observability or Layer 7 filtering.

Feature Comparison Summary

Feature Flannel Calico Cilium
Networking Mode Overlay (VXLAN/UDP) Native routing + Overlay eBPF native datapath
Network Policy Support Limited Full Kubernetes NetworkPolicy Extended L3-L7 NetworkPolicy
Performance Moderate (overlay overhead) High (native routing + eBPF acceleration) Very High (kernel-level eBPF)
Encryption No IPsec support Transparent encryption
Observability Basic logs Logs + metrics Rich with Hubble UI
Complexity Low Moderate High
Use Case Simplicity and compatibility Enterprise-grade security and scale Advanced security and observability

Best Practices and Recommendations

  • For development or smaller clusters, Flannel’s simplicity often suffices.
  • When network policy enforcement and scale are critical, Calico is the go-to.
  • Choose Cilium if you need advanced security policies, application-aware filtering, and deep observability with minimal latency.
  • Always test networking models in a staging environment to evaluate their fit with your infrastructure and workloads.
  • Keep cluster and CNI plugin versions up to date for security and performance improvements.

Conclusion

Understanding Kubernetes networking models empowers you to architect scalable, secure, and high-performing clusters. Flannel, Calico, and Cilium each offer unique benefits and trade-offs. Selecting the right CNI plugin depends on your specific needs around policy enforcement, visibility, and performance.

By aligning your networking strategy with your organizational goals, you can unlock Kubernetes’ full potential and deliver resilient, secure cloud native applications.