When localnet network attachment definitions (NADs) are created without specifying MTU, the MTU defaults to the overlay MTU. This is an incorrect default as with localnet there is no overlay. Using the overlay value of underlay - 100 bytes can lead to undesirable behavior for packets larger than 1400 bytes (for a 1500 byte MTU underlay) being sent to pods. The proper default for a localnet network should be the underlay MTU.

To handle upgrade (using 1500 byte underlay MTU as an example), we can detect which NADs existed without defaults before the default has been changed, and update the NADs to specify the previous defaulted MTU (1400). While new NADs will get the correct default (1500).

We should also update our documentation to make it clear what the MTU values will be for different network types. Note, the layer 2 network shows an example of setting the MTU, while localnet does not:

https://docs.redhat.com/en/documentation/openshift_container_platform/4.15/html/virtualization/networking#virt-creating-localnet-nad-cli_virt-connecting-vm-to-ovn-secondary-network

We specifically call out what these values will default to and how they should be used.