OpenShift Network Calculator

FAQ

• Q: What is HostPrefix?

  A: Defaults to 9 which means that a subnet of size /23 is allocated to each host; for example, given the default 10.128.0.0/14 cluster network, this will allocate 10.128.0.0/23, 10.128.2.0/23, 10.128.4.0/23, and so on. It controlls the number Pods/node.

• Q: What are Cluster Network, Service Network and Machine Network?

  A: ClusterNetwork is used by Pods, Service Network is for Services and MachineNetwork is the cluster node network.

• Q: I can see "number-of-nodes":{"want":512}. Do we really need 512 nodes?

  A: It is calculated by dividing the number of Pods by the Pods per nodes. That is, "number-of-pods": 262142 / "pods-per-node": 508 = "number-of-nodes": 512. However, "number-of-nodes":{"have":254} represents the number of nodes in the Machine Network.

• Q: How can we use this calulator effectively?

  A: This calculator offers the maximum number of Pods, Services, and nodes for the given network. However, it doesn't imply the obligation to utilize the entire capacity; these values merely represent the scalable size. To begin, you can calculate the number of nodes by dividing the required Pods by 500. This blog explains how we arrived at the value 500. Additionally, it can potentially go as high as 2500 under certain conditions, as indicated in this blog. To adjust the number of Pods in a node, modify the HostPrefix. The calculator also checks for network conflicts within the given network.

• Q: What is the reason behind network conflicts being exclusive to OVN-Kubernetes?

  A: The OVN-Kubernetes CNI exhibits network conflicts if the IP ranges 100.88.0.0/16, 100.64.0.0/16 and 169.254.169.0/29(InternalMasqueradeSubnet) are utilized. The OVN-Kubernetes uses these for the TransitSwitchSubnet and JoinSubnet. It is crucial to refrain from using these IP ranges in both the internal and external networks of the cluster to prevent conflicts. The JoinSubnet can be customized at the time of OVN-kubernetes migration by specifying the v4InternalSubnet spec under the spec.defaultNetwork.ovnKubernetesConfig object definition.

• Q: What does the InternalMasqueradeSubnet represent within the OVN-Kubernetes CNI?

  A: The internalMasqueradeSubnet contains the masquerade addresses in IPv4 CIDR format used internally by OVN-Kubernetes to enable host-to-service traffic. Each host in the cluster is configured with these addresses, as well as the shared gateway bridge interface. The values can be changed after installation. The subnet chosen should not overlap with other networks specified for OVN-Kubernetes as well as other networks used on the host. Additionally, the subnet must be large enough to accommodate 6 IPs (maximum prefix length /29). When omitted, this means no opinion and the platform is left to choose a reasonable default which is subject to change over time. The current default subnet is 169.254.169.0/29.