Also, it might help to understand how underlying containers work. For this reason, the move from Calico to Flannel needs to be properly planned and executed. Think of pods occasionally sending traffic to the wrong pod, losing connectivity, or terrible performance. The critical thing here is to clean up behind calico properly because otherwise, you might end up with weird non-deterministic behavior in the networking stack. Tell kubernetes what the new network range will be -cluster-cidr=10.244.0.0/16 and repeat with all master nodes, followed with all worker nodes (also you can just provision new workers and delete old ones – assuming your stateful workloads are not dependent on their host nodes). So to replace one CNI with another we simply need to install one, delete and clean up the other. CNIs can do way more, but this is the bare minimum required to have a functioning k8s cluster. It’s an abstraction through which each pod gets connected to every other pod, and everything gets its own unique network endpoint (eg. We are changing the main Container Network Interface which handles allocation and clean-up of network interfaces. We are replacing the main CNI plugin from Calico to Flannel.
In my case, it was simply that Calico was not playing nice with a bunch of multus-installed network interfaces that were required for connecting Kubevirt VMs to physical network infrastructure. Also, there might be some obscure edge cases that force you to do this kind of migration. As such it will probably showcase the CNI abstraction in a different light. But it’s nice little deep dive into kubernetes networking. It’s much more likely that you will simply provision a fresh cluster and retire the old one. This is not a common task in daily kubernetes administration. But to go from Calico to Flannel you will have to do most of the work manually. For instance, going from Flannel to Calico is almost trivial. So you want to change the underlying CNI provider in your kubernetes cluster? Well in some cases it’s as simple as running a few YAML manifests, in others, you have to do the hard work yourself.
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