64 Red Hat OpenShift and IBM Cloud Paks on IBM Power Systems: Volume 1
OpenShift provides the following SDN plug-ins for configuring the network:
/SM590000The ovs-subnet plug-in is the original plug-in, which provides a flat pod network in which 
every pod can communicate with every other pod and service. This configuration is the 
default configuration for single-tenant clusters. 
/SM590000The ovs-multitenant plug-in provides project-level isolation for pods and services. Each 
project receives a unique Virtual Network ID (VNID) that identifies traffic from pods that 
are assigned to the project. 
Pods from different projects cannot send packets to or receive packets from pods and 
services of a different project. However, projects that receive VNID 0 are more privileged, 
so they can communicate with all other pods, and all other pods can communicate with 
them. 
In OpenShift Container Platform clusters, the default project has VNID 0. This designation 
facilitates certain services, such as the load balancer, to communicate with all other pods 
in the cluster and vice versa.
/SM590000The ovs-networkpolicy plug-in allows project administrators to configure their own isolation 
policies by using NetworkPolicy objects.
OpenShift SDN maintains a registry of all nodes in the cluster. This registry is stored in etcd. 
When the system administrator registers a node, OpenShift SDN allocates an unused /23 
subnet from the cluster network and stores this subnet in the registry. When a node is 
removed or deleted from the cluster, the OpenShift SDN frees the corresponding cluster 
network subnet. This subnet becomes available for future allocations to new nodes.
This subnet value for nodes is specified in the Ansible inventory host variable 
osm_cluster_network_cidr. The default network if this variable is not set is 10.128.0.0./14 
(for example, 10.128.0.0 - 10.131.255.255).
The nodes are allocated /23 subnets (for example, 10.128.0.0/23, 10.128.2.0/23, 
10.128.4.0/23, and others). Therefore, the cluster network has 512 subnets that are available 
to assign to nodes, and a specific node is allocated 510 addresses that it can assign to the 
containers that are running on it. 
When the ovs-multitenant plug-in is used, the OpenShift SDN master also watches for the 
creation and deletion of projects and assigns VXLAN VNIDs to them. These VXLAN VNIDs 
are used later by the nodes to isolate traffic correctly.
To identify the cluster network subnet that is allocated to each node, run the oc get 
hostsubnet command with a user with cluster-admin privilege, as shown in Figure 4-3. 
Figure 4-3   Output from oc get hostsubnet command
Note: For more information about Open vSwitch, see this website.
# oc get hostsubnet
NAME                          HOST                          HOST IP        SUBNET         EGRESS CIDRS   EGRESS 
IPS
rbapp01.domain.example.com   rbapp01.domain.example.com   192.168.11.217   10.1.10.0/23   []             []
rbapp02.domain.example.com   rbapp02.domain.example.com   192.168.11.218   10.1.6.0/23    []             []
rbapp03.domain.example.com   rbapp03.domain.example.com   192.168.11.219   10.1.8.0/23    []             []
rbinf01.domain.example.com   rbinf01.domain.example.com   192.168.11.214   10.1.14.0/23   []             []
rbinf02.domain.example.com   rbinf02.domain.example.com   192.168.11.215   10.1.12.0/23   []             []
rbmst01.domain.example.com   rbmst01.domain.example.com   192.168.11.211   10.1.4.0/23    []             []
rbmst02.domain.example.com   rbmst02.domain.example.com   192.168.11.212   10.1.0.0/23    []             []
rbmst03.domain.example.com   rbmst03.domain.example.com   192.168.11.213   10.1.2.0/23    []             []