Chapter 11. Configuring the SDN

11.1. Overview

The OpenShift SDN enables communication between pods across the OpenShift cluster, establishing a pod network. Two SDN plug-ins are currently available (ovs-subnet and ovs-multitenant), which provide different methods for configuring the pod network.

For initial advanced installations, the ovs-subnet plug-in is installed and configured by default, though it can be overridden during installation using the os_sdn_network_plugin_name parameter.

For initial quick installations, the ovs-subnet plug-in is installed and configured by default as well, and can be reconfigured post-installation.

11.2. Configuring the Pod Network on Masters

Cluster administrators can control pod network settings on masters by modifying parameters in the networkConfig section of the master configuration file (located at /etc/origin/master/master-config.yaml by default): 

networkConfig:
  clusterNetworkCIDR: 10.1.0.0/16 1
  hostSubnetLength: 8 2
  networkPluginName: "redhat/openshift-ovs-subnet" 3
  serviceNetworkCIDR: 172.30.0.0/16 4
1
Cluster network for node IP allocation
2
Number of bits for pod IP allocation within a node
3
Set to redhat/openshift-ovs-subnet for the ovs-subnet plug-in or redhat/openshift-ovs-multitenant for the ovs-multitenant plug-in
4
Service IP allocation for the cluster
Important

The serviceNetworkCIDR and hostSubnetLength values cannot be changed after the cluster is first created, and clusterNetworkCIDR can only be changed to be a larger network that still contains the original network. For example, given the default value of 10.1.0.0/16, you could change clusterNetworkCIDR to 10.0.0.0/15 (i.e., 10.0.0.0/16 plus 10.1.0.0/16) but not to 10.2.0.0/16, because that does not overlap the original value.

11.3. Configuring the Pod Network on Nodes

Cluster administrators can control pod network settings on nodes by modifying parameters in the networkConfig section of the node configuration file (located at /etc/origin/node/node-config.yaml by default): 

networkConfig:
  mtu: 1450 1
  networkPluginName: "redhat/openshift-ovs-subnet" 2
1
Maximum transmission unit (MTU) for the pod overlay network
2
Set to redhat/openshift-ovs-subnet for the ovs-subnet plug-in or redhat/openshift-ovs-multitenant for the ovs-multitenant plug-in

11.4. Migrating Between SDN Plug-ins

If you are already using one SDN plug-in and want to switch to another:

  1. Change the networkPluginName parameter on all masters and nodes in their configuration files.
  2. Restart the atomic-openshift-master service on masters and the atomic-openshift-node service on nodes.

When switching from the ovs-subnet to the ovs-multitenant plug-in, all the existing projects in the cluster will be fully isolated (assigned unique VNIDs). Cluster administrators can choose to modify the project networks using the administrator CLI.

11.5. External Access to the Cluster Network

If a host that is external to OpenShift requires access to the cluster network, you have two options:

  1. Configure the host as an OpenShift node but mark it unschedulable so that the master does not schedule containers on it.
  2. Create a tunnel between your host and a host that is on the cluster network.

Both options are presented as part of a practical use-case in the documentation for configuring routing from an edge load-balancer to containers within OpenShift SDN.