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CNI: The Container Network Interface

Key Takeaways for AI & Readers
  • Fundamental Network Component: A CNI (Container Network Interface) plugin is essential for assigning IP addresses to Pods and enabling inter-Pod communication across nodes.
  • Overlay vs. Direct Routing: Overlay networks encapsulate traffic, offering portability but with potential MTU issues; direct routing (BGP) provides native performance but requires network support.
  • Performance Trade-offs: Overlay networks incur CPU overhead due to encapsulation, while direct routing offers lower latency and higher throughput.
  • CNI Choices: Modern CNIs like Cilium (eBPF-based) and Calico (flexible routing) offer advanced features, while Flannel remains a simple option for basic setups.

Every Kubernetes cluster needs a CNI Plugin. It is the software responsible for giving Pods IP addresses and allowing them to talk to each other across different Nodes.

1. Overlay vs. Direct Routing​

There are two primary ways Pods communicate over the network.

Node 1
Pod A
VXLAN Header
📦
Node 2
Pod B
Overlay networks wrap packets in another packet (Encapsulation). This allows pods to talk across different subnets but adds CPU overhead.

Overlay Networks (VXLAN / UDP)​

  • How: Packets are "wrapped" in another packet (like putting an envelope inside a larger envelope).
  • The "MTU" Headache: Because you are adding an extra header to every packet, the actual "Space for Data" (MTU) decreases. If your network isn't configured for this, packets get "fragmented," causing major performance drops.
  • Pros: Works on any cloud provider or subnet without complex network setup.
  • Cons: Higher CPU overhead due to constant wrapping/unwrapping.

Direct / BGP Routing​

  • How: Nodes act as routers and tell the physical network exactly where each Pod IP is.
  • Pros: Native performance, no encapsulation overhead.
  • Cons: Requires support from the underlying network/cloud (e.g. AWS VPC CNI).
  • Examples: Cilium, Calico (BGP mode).

2. Which CNI should I choose?​

  1. Cilium: The modern choice. Uses eBPF for high performance and deep observability.
  2. Calico: The battle-tested choice. Supports both Overlay and BGP routing.
  3. Flannel: The "Keep It Simple" choice. Good for small labs.