Cilium CIDR overlap issue

We had faced this issue in our AWS EKS cluster; I wrote an article about that incident. See it here. CIDR Conflict When Moving from AWS CNI to Cilium

TL;DR

This entire debugging experience reminded us how low-level networking issues can surface in unexpected ways sometimes months after a change is made.

If you’re using Cilium in a cloud environment like EKS:

• Always define non-overlapping IP ranges for Cilium’s IPAM.
• Be cautious with large CIDRs like 10.0.0.0/8.
• Use tools like Hubble and Wireshark in combination to debug packet flows.
• Document your VPC-wide CIDR usage especially when you use multiple clusters or peered networks.

Hope this post helps you avoid days of frustration and maybe even saves your cluster someday.