The implicit default for MTU with OVN localnet is 1400. That is counterintuitive for network-savvy users who expect the default to be 1500. That leads to situations when a customer observes packet loss due to fragmentation, but is unable to spot an invalid configuration.

4.15+ (not entirely sure when was the localnet UI introduced)

Always

1. Create a new NAD with the localnet topology, without setting any MTU
2. Look at the YAML

...
spec:
  config: |-
    {
        "cniVersion": "0.4.0",
        "name": "physname",
        "type": "ovn-k8s-cni-overlay",
        "netAttachDefName": "phoracek/net1",
        "topology": "localnet"
    }

...
spec:
  config: |-
    {
        "cniVersion": "0.4.0",
        "name": "physname",
        "type": "ovn-k8s-cni-overlay",
        "netAttachDefName": "phoracek/net1",
        "mtu": 1500,
        "topology": "localnet"
    }

The UI should require the MTU attribute to be set explicitly, defaulting it text field to 1500.