How should I approach writing a simple CNI plugin for learning?

For a practical learning path, study a small, established implementation rather than starting from an advanced project. A basic VXLAN-based network is a good example because it shows how nodes form tunnels and route pod traffic between one another. The CNI interface itself is fairly small, so a minimal plugin can be written quickly; most of the interesting work is understanding Linux networking, IPAM, routing, lifecycle cleanup, and multi-node behavior.

eBPF is worth learning if your long-term goal is to understand modern high-performance Kubernetes networking, but it’s better as a later step. First get ordinary Linux networking working with namespaces, veth pairs, bridges or routes, and packet forwarding. Then add eBPF at the traffic-control or other kernel hooks and compare what it replaces.

Start with the fundamentals instead of trying to reproduce a full-featured networking project immediately. On a single node, learn how to assign pod IPs, create a veth pair, configure routes, and implement a small IP address manager. Once that works, expand to multiple nodes with static routes. Only after those pieces make sense should you explore eBPF, such as attaching programs at Linux traffic-control hooks.