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Changing the MTU for the cluster network

As a cluster administrator, you can change the MTU for the cluster network after cluster installation. This change is disruptive as cluster nodes must be rebooted to finalize the MTU change. You can change the MTU only for clusters using the OVN-Kubernetes or OpenShift SDN network plugins.

About the cluster MTU

During installation the maximum transmission unit (MTU) for the cluster network is detected automatically based on the MTU of the primary network interface of nodes in the cluster. You do not normally need to override the detected MTU.

You might want to change the MTU of the cluster network for several reasons:

  • The MTU detected during cluster installation is not correct for your infrastructure

  • Your cluster infrastructure now requires a different MTU, such as from the addition of nodes that need a different MTU for optimal performance

You can change the cluster MTU for only the OVN-Kubernetes and OpenShift SDN cluster network plugins.

Service interruption considerations

When you initiate an MTU change on your cluster the following effects might impact service availability:

  • At least two rolling reboots are required to complete the migration to a new MTU. During this time, some nodes are not available as they restart.

  • Specific applications deployed to the cluster with shorter timeout intervals than the absolute TCP timeout interval might experience disruption during the MTU change.

MTU value selection

When planning your MTU migration there are two related but distinct MTU values to consider.

  • Hardware MTU: This MTU value is set based on the specifics of your network infrastructure.

  • Cluster network MTU: This MTU value is always less than your hardware MTU to account for the cluster network overlay overhead. The specific overhead is determined by your network plugin:

    • OVN-Kubernetes: 100 bytes

    • OpenShift SDN: 50 bytes

If your cluster requires different MTU values for different nodes, you must subtract the overhead value for your network plugin from the lowest MTU value that is used by any node in your cluster. For example, if some nodes in your cluster have an MTU of 9001, and some have an MTU of 1500, you must set this value to 1400.

To avoid selecting an MTU value that is not acceptable by a node, verify the maximum MTU value (maxmtu) that is accepted by the network interface by using the ip -d link command.

How the migration process works

The following table summarizes the migration process by segmenting between the user-initiated steps in the process and the actions that the migration performs in response.

Table 1. Live migration of the cluster MTU
User-initiated steps OpenShift Container Platform activity

Set the following values in the Cluster Network Operator configuration:

  • spec.migration.mtu.machine.to

  • spec.migration.mtu.network.from

  • spec.migration.mtu.network.to

Cluster Network Operator (CNO): Confirms that each field is set to a valid value.

  • The mtu.machine.to must be set to either the new hardware MTU or to the current hardware MTU if the MTU for the hardware is not changing. This value is transient and is used as part of the migration process. Separately, if you specify a hardware MTU that is different from your existing hardware MTU value, you must manually configure the MTU to persist by other means, such as with a machine config, DHCP setting, or a Linux kernel command line.

  • The mtu.network.from field must equal the network.status.clusterNetworkMTU field, which is the current MTU of the cluster network.

  • The mtu.network.to field must be set to the target cluster network MTU and must be lower than the hardware MTU to allow for the overlay overhead of the network plugin. For OVN-Kubernetes, the overhead is 100 bytes and for OpenShift SDN the overhead is 50 bytes.

If the values provided are valid, the CNO writes out a new temporary configuration with the MTU for the cluster network set to the value of the mtu.network.to field.

Machine Config Operator (MCO): Performs a rolling reboot of each node in the cluster.

Reconfigure the MTU of the primary network interface for the nodes on the cluster. You can use a variety of methods to accomplish this, including:

  • Deploying a new NetworkManager connection profile with the MTU change

  • Changing the MTU through a DHCP server setting

  • Changing the MTU through boot parameters

N/A

Set the mtu value in the CNO configuration for the network plugin and set spec.migration to null.

Machine Config Operator (MCO): Performs a rolling reboot of each node in the cluster with the new MTU configuration.

Changing the cluster MTU

As a cluster administrator, you can change the maximum transmission unit (MTU) for your cluster. The migration is disruptive and nodes in your cluster might be temporarily unavailable as the MTU update rolls out.

The following procedure describes how to change the cluster MTU by using either machine configs, DHCP, or an ISO. If you use the DHCP or ISO approach, you must refer to configuration artifacts that you kept after installing your cluster to complete the procedure.

Prerequisites

  • You installed the OpenShift CLI (oc).

  • You are logged in to the cluster with a user with cluster-admin privileges.

  • You identified the target MTU for your cluster. The correct MTU varies depending on the network plugin that your cluster uses:

    • OVN-Kubernetes: The cluster MTU must be set to 100 less than the lowest hardware MTU value in your cluster.

    • OpenShift SDN: The cluster MTU must be set to 50 less than the lowest hardware MTU value in your cluster.

Procedure

To increase or decrease the MTU for the cluster network complete the following procedure.

  1. To obtain the current MTU for the cluster network, enter the following command:

    $ oc describe network.config cluster
    Example output
    ...
Status:
  Cluster Network:
    Cidr:               10.217.0.0/22
    Host Prefix:        23
  Cluster Network MTU:  1400
  Network Type:         OpenShiftSDN
  Service Network:
    10.217.4.0/23
...

  2. Prepare your configuration for the hardware MTU:

    • If your hardware MTU is specified with DHCP, update your DHCP configuration such as with the following dnsmasq configuration:

      dhcp-option-force=26,<mtu>

      where:

      <mtu>

      Specifies the hardware MTU for the DHCP server to advertise.

    • If your hardware MTU is specified with a kernel command line with PXE, update that configuration accordingly.

    • If your hardware MTU is specified in a NetworkManager connection configuration, complete the following steps. This approach is the default for OpenShift Container Platform if you do not explicitly specify your network configuration with DHCP, a kernel command line, or some other method. Your cluster nodes must all use the same underlying network configuration for the following procedure to work unmodified.

      1. Find the primary network interface:

        • If you are using the OpenShift SDN network plugin, enter the following command:

          $ oc debug node/<node_name> -- chroot /host ip route list match 0.0.0.0/0 | awk '{print $5 }'

          where:

          <node_name>

          Specifies the name of a node in your cluster.

        • If you are using the OVN-Kubernetes network plugin, enter the following command:

          $ oc debug node/<node_name> -- chroot /host nmcli -g connection.interface-name c show ovs-if-phys0

          where:

          <node_name>

          Specifies the name of a node in your cluster.

      2. Create the following NetworkManager configuration in the <interface>-mtu.conf file:

        Example NetworkManager connection configuration
        [connection-<interface>-mtu]
match-device=interface-name:<interface>
ethernet.mtu=<mtu>

        where:

        <mtu>

        Specifies the new hardware MTU value.

        <interface>

        Specifies the primary network interface name.

      3. Create two MachineConfig objects, one for the control plane nodes and another for the worker nodes in your cluster:

        1. Create the following Butane config in the control-plane-interface.bu file:

          variant: openshift
version: 4.12.0
metadata:
  name: 01-control-plane-interface
  labels:
    machineconfiguration.openshift.io/role: master
storage:
  files:
    - path: /etc/NetworkManager/conf.d/99-<interface>-mtu.conf (1)
      contents:
        local: <interface>-mtu.conf (2)
      mode: 0600
          1 Specify the NetworkManager connection name for the primary network interface.
          2 Specify the local filename for the updated NetworkManager configuration file from the previous step.

        2. Create the following Butane config in the worker-interface.bu file:

          variant: openshift
version: 4.12.0
metadata:
  name: 01-worker-interface
  labels:
    machineconfiguration.openshift.io/role: worker
storage:
  files:
    - path: /etc/NetworkManager/conf.d/99-<interface>-mtu.conf (1)
      contents:
        local: <interface>-mtu.conf (2)
      mode: 0600
          1 Specify the NetworkManager connection name for the primary network interface.
          2 Specify the local filename for the updated NetworkManager configuration file from the previous step.

        3. Create MachineConfig objects from the Butane configs by running the following command:

          $ for manifest in control-plane-interface worker-interface; do
    butane --files-dir . $manifest.bu > $manifest.yaml
  done

  3. To begin the MTU migration, specify the migration configuration by entering the following command. The Machine Config Operator performs a rolling reboot of the nodes in the cluster in preparation for the MTU change.

    $ oc patch Network.operator.openshift.io cluster --type=merge --patch \
  '{"spec": { "migration": { "mtu": { "network": { "from": <overlay_from>, "to": <overlay_to> } , "machine": { "to" : <machine_to> } } } } }'

    where:

    <overlay_from>

    Specifies the current cluster network MTU value.

    <overlay_to>

    Specifies the target MTU for the cluster network. This value is set relative to the value for <machine_to> and for OVN-Kubernetes must be 100 less and for OpenShift SDN must be 50 less.

    <machine_to>

    Specifies the MTU for the primary network interface on the underlying host network.

    Example that increases the cluster MTU
    $ oc patch Network.operator.openshift.io cluster --type=merge --patch \
  '{"spec": { "migration": { "mtu": { "network": { "from": 1400, "to": 9000 } , "machine": { "to" : 9100} } } } }'

  4. As the MCO updates machines in each machine config pool, it reboots each node one by one. You must wait until all the nodes are updated. Check the machine config pool status by entering the following command:

    $ oc get mcp

    A successfully updated node has the following status: UPDATED=true, UPDATING=false, DEGRADED=false.

    By default, the MCO updates one machine per pool at a time, causing the total time the migration takes to increase with the size of the cluster.

  5. Confirm the status of the new machine configuration on the hosts:

    1. To list the machine configuration state and the name of the applied machine configuration, enter the following command:

      $ oc describe node | egrep "hostname|machineconfig"
      Example output
      kubernetes.io/hostname=master-0
machineconfiguration.openshift.io/currentConfig: rendered-master-c53e221d9d24e1c8bb6ee89dd3d8ad7b
machineconfiguration.openshift.io/desiredConfig: rendered-master-c53e221d9d24e1c8bb6ee89dd3d8ad7b
machineconfiguration.openshift.io/reason:
machineconfiguration.openshift.io/state: Done

      Verify that the following statements are true:

      • The value of machineconfiguration.openshift.io/state field is Done.

      • The value of the machineconfiguration.openshift.io/currentConfig field is equal to the value of the machineconfiguration.openshift.io/desiredConfig field.

    2. To confirm that the machine config is correct, enter the following command:

      $ oc get machineconfig <config_name> -o yaml | grep ExecStart

      where <config_name> is the name of the machine config from the machineconfiguration.openshift.io/currentConfig field.

      The machine config must include the following update to the systemd configuration:

      ExecStart=/usr/local/bin/mtu-migration.sh

  6. Update the underlying network interface MTU value:

    • If you are specifying the new MTU with a NetworkManager connection configuration, enter the following command. The MachineConfig Operator automatically performs a rolling reboot of the nodes in your cluster.

      $ for manifest in control-plane-interface worker-interface; do
    oc create -f $manifest.yaml
  done

    • If you are specifying the new MTU with a DHCP server option or a kernel command line and PXE, make the necessary changes for your infrastructure.

  7. As the MCO updates machines in each machine config pool, it reboots each node one by one. You must wait until all the nodes are updated. Check the machine config pool status by entering the following command:

    $ oc get mcp

    A successfully updated node has the following status: UPDATED=true, UPDATING=false, DEGRADED=false.

    By default, the MCO updates one machine per pool at a time, causing the total time the migration takes to increase with the size of the cluster.

  8. Confirm the status of the new machine configuration on the hosts:

    1. To list the machine configuration state and the name of the applied machine configuration, enter the following command:

      $ oc describe node | egrep "hostname|machineconfig"
      Example output
      kubernetes.io/hostname=master-0
machineconfiguration.openshift.io/currentConfig: rendered-master-c53e221d9d24e1c8bb6ee89dd3d8ad7b
machineconfiguration.openshift.io/desiredConfig: rendered-master-c53e221d9d24e1c8bb6ee89dd3d8ad7b
machineconfiguration.openshift.io/reason:
machineconfiguration.openshift.io/state: Done

      Verify that the following statements are true:

      • The value of machineconfiguration.openshift.io/state field is Done.

      • The value of the machineconfiguration.openshift.io/currentConfig field is equal to the value of the machineconfiguration.openshift.io/desiredConfig field.

    2. To confirm that the machine config is correct, enter the following command:

      $ oc get machineconfig <config_name> -o yaml | grep path:

      where <config_name> is the name of the machine config from the machineconfiguration.openshift.io/currentConfig field.

      If the machine config is successfully deployed, the previous output contains the /etc/NetworkManager/conf.d/99-<interface>-mtu.conf file path and the ExecStart=/usr/local/bin/mtu-migration.sh line.

  9. To finalize the MTU migration, enter one of the following commands:

    • If you are using the OVN-Kubernetes network plugin:

      $ oc patch Network.operator.openshift.io cluster --type=merge --patch \
  '{"spec": { "migration": null, "defaultNetwork":{ "ovnKubernetesConfig": { "mtu": <mtu> }}}}'

      where:

      <mtu>

      Specifies the new cluster network MTU that you specified with <overlay_to>.

    • If you are using the OpenShift SDN network plugin:

      $ oc patch Network.operator.openshift.io cluster --type=merge --patch \
  '{"spec": { "migration": null, "defaultNetwork":{ "openshiftSDNConfig": { "mtu": <mtu> }}}}'

      where:

      <mtu>

      Specifies the new cluster network MTU that you specified with <overlay_to>.

  10. After finalizing the MTU migration, each MCP node is rebooted one by one. You must wait until all the nodes are updated. Check the machine config pool status by entering the following command:

    $ oc get mcp

    A successfully updated node has the following status: UPDATED=true, UPDATING=false, DEGRADED=false.

Verification

You can verify that a node in your cluster uses an MTU that you specified in the previous procedure.

  1. To get the current MTU for the cluster network, enter the following command:

    $ oc describe network.config cluster

  2. Get the current MTU for the primary network interface of a node.

    1. To list the nodes in your cluster, enter the following command:

      $ oc get nodes

    2. To obtain the current MTU setting for the primary network interface on a node, enter the following command:

      $ oc debug node/<node> -- chroot /host ip address show <interface>

      where:

      <node>

      Specifies a node from the output from the previous step.

      <interface>

      Specifies the primary network interface name for the node.

      Example output
      ens3: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 8051

Additional resources