Red Hat SummitChapter 22. CIDR range definitions
If your cluster uses OVN-Kubernetes, you must specify non-overlapping ranges for Classless Inter-Domain Routing (CIDR) subnet ranges.
The following subnet types are mandatory for a cluster that uses OVN-Kubernetes:
- Join: Uses a join switch to connect gateway routers to distributed routers. A join switch reduces the number of IP addresses for a distributed router. For a cluster that uses the OVN-Kubernetes plugin, an IP address from a dedicated subnet is assigned to any logical port that attaches to the join switch.
- Masquerade: Prevents collisions for identical source and destination IP addresses that are sent from a node as hairpin traffic to the same node after a load balancer makes a routing decision.
- Transit: A transit switch is a type of distributed switch that spans across all nodes in the cluster. A transit switch routes traffic between different zones. For a cluster that uses the OVN-Kubernetes plugin, an IP address from a dedicated subnet is assigned to any logical port that attaches to the transit switch.
You can change the join and transit CIDR ranges for your cluster as a post-installation task.
OVN-Kubernetes, the default network provider in OpenShift Container Platform 4.14 and later versions, internally uses the following IP address subnet ranges:
-
V4JoinSubnet:100.64.0.0/16 -
V6JoinSubnet:fd98::/64 -
V4TransitSwitchSubnet:100.88.0.0/16 -
V6TransitSwitchSubnet:fd97::/64 -
defaultV4MasqueradeSubnet:169.254.169.0/29 -
defaultV6MasqueradeSubnet:fd69::/125
The earlier list includes join, transit, and masquerade IPv4 and IPv6 address subnets. If your cluster uses OVN-Kubernetes, do not include any of these IP address subnet ranges in any other CIDR definitions in your cluster or infrastructure.
22.1. Machine CIDR
In the Machine classless inter-domain routing (CIDR) field, you must specify the IP address range for machines or cluster nodes.
You cannot change Machine CIDR ranges after you created your cluster.
The default is 10.0.0.0/16. This range must not conflict with any connected networks.
22.2. Service CIDR
In the Service CIDR field, you must specify the IP address range for services. The range must be large enough to accommodate your workload. The address block must not overlap with any external service accessed from within the cluster. The default is 172.30.0.0/16.
22.3. Pod CIDR
In the pod CIDR field, you must specify the IP address range for pods.
The pod CIDR is the same as the clusterNetwork CIDR and the cluster CIDR. The range must be large enough to accommodate your workload. The address block must not overlap with any external service accessed from within the cluster. The default is 10.128.0.0/14. You can expand the range after cluster installation.
22.4. Host prefix
In the hostPrefix parameter, you must specify the subnet prefix length assigned to pods scheduled to individual machines. The host prefix determines the pod IP address pool for each machine.
For example, if the host prefix is set to /23, each machine is assigned a /23 subnet from the pod CIDR address range. The default is /23, allowing 510 cluster nodes, and 510 pod IP addresses per node.
Consider another example where you set the clusterNetwork.cidr parameter to 10.128.0.0/16, you define the complete address space for the cluster. This assigns a pool of 65536 IP addresses to your cluster. If you then set the hostPrefix parameter to /23, you define a subnet slice to each node in the cluster, where the /23 slice becomes a subnet of the /16 subnet network. This assigns 512 IP addresses to each node, where 2 IP addresses get reserved for networking and broadcasting purposes. The following example calculation uses these IP address figures to determine the maximum number of nodes that you can create for your cluster:
65536 / 512 = 128
65536 / 512 = 128You can use the Red Hat OpenShift Network Calculator to calculate the maximum number of nodes for your cluster.
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