Configuring MTU

Depending on the environment Calico is being deployed into it may be helpful or even necessary to configure the MTU of the veth (or TAP) that is attached to each workload and the tunnel devices if IP-in-IP is enabled.

Selecting MTU size

Typically the MTU for your workload interfaces should match the network MTU. If you need IP-in-IP then the MTU size for both the workload and tunnel interfaces should be 20 bytes less than the network MTU for your network. This is due to the extra 20 byte header that the tunnel will add to each packet.

Common MTU sizes

Default MTU sizes

The default MTU for workload interfaces is 1500, this is to match the most common network MTU size. The default MTU for the IP-in-IP tunnel device is 1440 to match the value needed in GCE.

Using flannel for networking

When using flannel for networking, the MTU for the network interfaces should match the MTU of the flannel interface. In the above table the 4th column “Calico MTU with VXLAN” is the expected MTU when using flannel configured with VXLAN.

MTU configuration

It is the job of the network plugin to create new interfaces, the current major plugins are CNI and libnetwork. Currently Docker and the Mesos Docker Containerizer integration use libnetwork.

CNI, which is used by Kubernetes and the Mesos Unified Containerizer, supports configuring the MTU through the CNI configuration file.

The user will also want to configure Calico’s IP-in-IP interface MTU when IP-in-IP is enabled on the cluster. Refer to the MTU table at the top of the page to choose the value that matches your environment.

Note: The MTU on existing workloads will not be updated with these changes. To update workload MTUs, see the section that corresponds to your plugin type.

MTU configuration with libnetwork

The MTU of the veth pairs created by the Calico libnetwork plugin can be configured by setting the CALICO_LIBNETWORK_VETH_MTU environment variable on the libnetwork process.

This should either be set on the standalone libnetwork service, or on the calico/node container as a whole.

MTU configuration with CNI

MTU is set in the by the "mtu": <MTU size> field of the CNI configuration. Example:

  {
    "name": "any_name",
    "cniVersion": "0.1.0",
    "type": "calico",
    "mtu": 1480,
    "ipam": {
      "type": "calico-ipam"
    }
  }

Note: If using Kubernetes self-hosted manifests, you should modify the veth_mtu value in the Calico ConfigMap instead, and leave "mtu" here set to __CNI_MTU__. See below for more details.

MTU configuration with Kubernetes self-hosted manifests

When using Kubernetes self-hosted manifests, the CNI plugin derives the MTU value from the veth_mtu field of the calico-config ConfigMap, and it is set to 1440 by default. On restart of the calico/node pods, any references to __CNI_MTU__ are replaced by the veth_mtu value and inserted into the CNI configuration file (aka conflist) at the directory specified by Kubernetes (currently defaults to /etc/cni/net.d/).

Restarting the calico/node pods will also update any Calico tunnel network interfaces on that node. From this point forward, any pods started will also have the updated MTU value.

Setting tunnel MTU with a Felix environment variable

Passing in the environment variable FELIX_IPINIPMTU when running the calico/node container will set the MTU for Felix to use.

When using the Kubernetes self-hosted manifests, Felix derives this value from the veth_mtu field of the calico-config ConfigMap, which is set to 1440 by default.

Setting tunnel MTU with calicoctl

To set the IP-in-IP MTU value for all Calico nodes in your cluster, use the following command to retrieve the current Felix settings.

calicoctl get felixconfig --export -o yaml > felix.yaml

Modify ipipMTU to the intended integer value.

vim felix.yaml

Replace the current felixconfig settings.

calicoctl replace -f felix.yaml

Note: Setting the ipipMTU config option will result in an immediate update of the tunnel interface MTU on all of the active nodes in your cluster.