Flannel is K3s’s default CNI plugin, and while it works fine for many use cases, it’s not always the best choice. You might need an alternative if you require advanced networking features like network policies, specific routing configurations, or better performance for large clusters.
Let’s see what happens when we replace Flannel with Calico.
Here’s a K3s cluster running with Flannel, and then we’ll switch it over to Calico.
# On your K3s server node
curl -sfL https://get.k3s.io | sh -s - --disable traefik --disable local-storage
# Verify Flannel is running
kubectl get pods -n kube-system | grep flanel
# Output will show flanel pods running
# Now, let's uninstall Flannel
kubectl delete daemonset flannel -n kube-system
# Install Calico
# Download the Calico manifest
curl -O https://raw.githubusercontent.com/projectcalico/calico/v3.26.1/manifests/calico.yaml
# Apply the Calico manifest
kubectl apply -f calico.yaml
# Verify Calico is running
kubectl get pods -n kube-system | grep calico
# Output will show calico-node pods running
# Verify pods can communicate across nodes
# Assuming you have pods on different nodes, test connectivity
# (This part would involve deploying sample pods and testing ping/curl between them)
The core problem Flannel solves is enabling pods on different nodes in your K3s cluster to communicate with each other. It does this by creating a virtual network overlay. When you replace it, you’re essentially swapping out this overlay mechanism for a different one.
The most common reason to switch from Flannel to an alternative CNI like Calico is the need for network policy enforcement. Flannel, by default, doesn’t provide a robust way to define and enforce security policies between pods. Calico, on the other hand, is built with network policy as a first-class citizen, allowing you to restrict traffic flow based on labels, namespaces, and other Kubernetes objects.
Another frequent driver is performance and scalability. For very large clusters or high-throughput networking requirements, some CNIs might offer better performance characteristics. This could be due to their underlying implementation (e.g., using native Linux networking features like iptables or eBPF directly rather than an overlay).
Specific routing requirements can also lead to a CNI change. If your network topology is complex, or you need fine-grained control over how pod traffic is routed, a more advanced CNI might be necessary. Flannel’s default VXLAN or UDP encapsulation can be limiting in such scenarios.
Operational simplicity for certain tasks can be a factor. While Flannel is simple to get started with, managing advanced features or troubleshooting complex network issues might be easier with a CNI that offers more visibility and control, like Calico or Cilium.
Finally, integration with existing infrastructure might dictate your choice. If your data center already relies on specific networking technologies or management tools, choosing a CNI that integrates well with those can simplify deployment and maintenance.
When replacing Flannel, you’re not just installing a new CNI; you’re fundamentally changing how Kubernetes networking operates. The key is to ensure the new CNI is compatible with your Kubernetes version and that your underlying network infrastructure can support its requirements.
The next error you’ll likely encounter after a successful CNI switch is related to service discovery or DNS resolution, as the CNI plays a role in how kube-dns or CoreDNS is configured and how pods can reach it.