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Chapter 5. CIDR range definitions
If your cluster uses OVN-Kubernetes, you must specify non-overlapping ranges for Classless Inter-Domain Routing (CIDR) subnet ranges.
For Red Hat OpenShift Service on AWS 4.17 and later versions, clusters use 169.254.0.0/17 for IPv4 and fd69::/112 for IPv6 as the default masquerade subnet. These ranges should also be avoided by users. For upgraded clusters, there is no change to the default masquerade subnet.
The following subnet types and 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, masquerade, and transit CIDR ranges for your cluster as a post-installation task.
When specifying subnet CIDR ranges, ensure that the subnet CIDR range is within the defined Machine CIDR. You must verify that the subnet CIDR ranges allow for enough IP addresses for all intended workloads depending on which platform the cluster is hosted.
OVN-Kubernetes, the default network provider in Red Hat OpenShift Service on AWS 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:169.254.0.0/17 -
defaultV6MasqueradeSubnet:fd69::/112
The previous 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.
5.1. Machine CIDR
In the Machine classless inter-domain routing (CIDR) field, you must specify the IP address range for machines or cluster nodes.
Machine CIDR ranges cannot be changed after creating your cluster.
This range must encompass all CIDR address ranges for your virtual private cloud (VPC) subnets. Subnets must be contiguous. A minimum IP address range of 128 addresses, using the subnet prefix /25, is supported for single availability zone deployments. A minimum address range of 256 addresses, using the subnet prefix /24, is supported for deployments that use multiple availability zones.
The default is 10.0.0.0/16. This range must not conflict with any connected networks.
When using ROSA with HCP, the static IP address 172.20.0.1 is reserved for the internal Kubernetes API address. The machine, pod, and service CIDRs ranges must not conflict with this IP address.
5.2. Service CIDR
In the Service CIDR field, you must specify the IP address range for services. It is recommended, but not required, that the address block is the same between clusters. This will not create IP address conflicts. 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.
5.3. Pod CIDR
In the pod CIDR field, you must specify the IP address range for pods.
It is recommended, but not required, that the address block is the same between clusters. This will not create IP address conflicts. 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.
5.4. Host Prefix
In the Host Prefix field, 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 512 cluster nodes, and 512 pods per node (both of which are beyond our maximum supported).
