kubernetes set service cidr and pod cidr the same

ghz 7months ago ⋅ 191 views

When we use kubeadm to set up a k8s cluster, there are two options to config:

--pod-network-cidr

--service-cidr (default ‘10.96.0.0/12’)

Question is:

  1. If I use 10.244.0.0./12 for pod-network-cidr, do I need to save that IP range for Kubernetes? What happens if we already start to use 10.244.0.0/12 for other machines.
  2. Can I set service-cidr and the pod-network-cidr the same range? I don't understand how service-cidr works.

Answers

When setting up a Kubernetes cluster with kubeadm, it's important to correctly configure both the --pod-network-cidr and --service-cidr options to avoid conflicts and ensure smooth operation of the cluster. Let's break down the details and implications of these configurations:

--pod-network-cidr

The --pod-network-cidr specifies the IP address range for Pods within the cluster. Each pod gets an IP address from this range. For example, if you set --pod-network-cidr=10.244.0.0/16, the network range 10.244.0.0/16 will be used to assign IP addresses to pods.

Considerations for --pod-network-cidr:

  • Unique Network Range: The IP range specified should not overlap with any IP ranges used by other networks or machines in your environment. This is to prevent IP conflicts. If you already have machines using 10.244.0.0/12, you should choose a different CIDR for the pod network.
  • Reserved for Kubernetes: This range should be reserved exclusively for Kubernetes to manage pod IPs. No other services or machines should use this range to avoid conflicts.

--service-cidr

The --service-cidr specifies the IP address range from which Kubernetes will allocate cluster IPs for Services. Services in Kubernetes are abstract ways to expose an application running on a set of Pods as a network service.

Considerations for --service-cidr:

  • Separate from Pod Network: The service CIDR should be different from the pod network CIDR to avoid conflicts. Services and Pods should have distinct IP ranges.
  • Default Range: The default service CIDR is 10.96.0.0/12, but you can customize it. Ensure that this range does not overlap with any existing network ranges in your environment.

Why Separate Ranges?

The pod network CIDR and the service CIDR must be separate because they serve different purposes:

  • Pod Network CIDR: Used for assigning IP addresses to individual pods. These addresses are used for internal communication between pods.
  • Service CIDR: Used for assigning IP addresses to services, which provide stable endpoints to access a set of pods. These addresses are used by clients (both within and outside the cluster) to access services.

Example Configuration

Here's how you might configure a Kubernetes cluster with kubeadm:

  1. Choose Non-Conflicting Ranges: Ensure that the ranges do not overlap with any existing networks in your environment.

  2. Initialize Cluster with kubeadm:

    kubeadm init --pod-network-cidr=10.244.0.0/16 --service-cidr=10.96.0.0/12

  3. Apply Network Plugin: After initializing the cluster, you need to apply a network plugin (e.g., Calico, Flannel) that supports the pod network CIDR you specified. For Flannel:

    kubectl apply -f https://raw.githubusercontent.com/coreos/flannel/master/Documentation/kube-flannel.yml

Conclusion

  • Avoid Overlap: Ensure that the pod network CIDR and service CIDR do not overlap with each other or with any other IP ranges used in your network environment.
  • Separate Responsibilities: Understand that the pod network CIDR is for pods, and the service CIDR is for services, and configure them accordingly.
  • Network Plugin Compatibility: Make sure the network plugin you choose supports the pod network CIDR you configure.

By following these guidelines, you can ensure a properly configured and functional Kubernetes cluster without IP conflicts.

#kubeadm