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Chapter 9. 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 cluster network providers.
9.1. 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 providers.
9.1.1. 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.
9.1.2. 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 cluster network provider:
-
OVN-Kubernetes:
100
bytes -
OpenShift SDN:
50
bytes
-
OVN-Kubernetes:
If your cluster requires different MTU values for different nodes, you must subtract the overhead value for your cluster network provider 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
.
9.1.3. 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.
User-initiated steps | OpenShift Container Platform activity |
---|---|
Set the following values in the Cluster Network Operator configuration:
| Cluster Network Operator (CNO): Confirms that each field is set to a valid value.
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 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:
| N/A |
Set the | Machine Config Operator (MCO): Performs a rolling reboot of each node in the cluster with the new MTU configuration. |
9.2. 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 cluster network provider 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.
-
OVN-Kubernetes: The cluster MTU must be set to
Procedure
To increase or decrease the MTU for the cluster network complete the following procedure.
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 ...
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.
Find the primary network interface:
If you are using the OpenShift SDN cluster network provider, 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 cluster network provider, 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.
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.
Create two
MachineConfig
objects, one for the control plane nodes and another for the worker nodes in your cluster:Create the following Butane config in the
control-plane-interface.bu
file:variant: openshift version: 4.10.0 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
Create the following Butane config in the
worker-interface.bu
file:variant: openshift version: 4.10.0 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
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
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 be100
less and for OpenShift SDN must be50
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} } } } }'
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
.NoteBy 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.
Confirm the status of the new machine configuration on the hosts:
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 isDone
. -
The value of the
machineconfiguration.openshift.io/currentConfig
field is equal to the value of themachineconfiguration.openshift.io/desiredConfig
field.
-
The value of
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 themachineconfiguration.openshift.io/currentConfig
field.The machine config must include the following update to the systemd configuration:
ExecStart=/usr/local/bin/mtu-migration.sh
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.
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
.NoteBy 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.
Confirm the status of the new machine configuration on the hosts:
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 isDone
. -
The value of the
machineconfiguration.openshift.io/currentConfig
field is equal to the value of themachineconfiguration.openshift.io/desiredConfig
field.
-
The value of
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 themachineconfiguration.openshift.io/currentConfig
field.If the machine config is successfully deployed, the previous output contains the
/etc/NetworkManager/system-connections/<connection_name>
file path.The machine config must not contain the
ExecStart=/usr/local/bin/mtu-migration.sh
line.
To finalize the MTU migration, enter one of the following commands:
If you are using the OVN-Kubernetes cluster network provider:
$ 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 cluster network provider:
$ 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>
.
Verification
You can verify that a node in your cluster uses an MTU that you specified in the previous procedure.
To get the current MTU for the cluster network, enter the following command:
$ oc describe network.config cluster
Get the current MTU for the primary network interface of a node.
To list the nodes in your cluster, enter the following command:
$ oc get nodes
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