Comment 3 for bug 1899487

That is an excellent question, I see that the example provided in the Nova documentation [0] provides `null` for the MTU. There are also a Nova bug 1746323 on the lack of actual API documentation for the OpenStack format metadata, so I guess they could expect no less than diverging implementations consuming it in the wild.

However, I also see that the reporting of `null` MTU was fixed [1] on the back of Nova bug 1576713 a few years back so that it now provides an actual MTU regardless of network addressing type.

The OpenStack format metadata does provide a separate field that distinguishes between the various types of dynamic and static configuration [2] and I see that cloud-init already makes use of it [3].

So I would suggest that whenever OpenStack eludes to dynamic configuration being in play cloud-init should not write the MTU value into the on-disk configuration but let it be configured by dynamic network configuration protocol.

What do you think?

0: https://docs.openstack.org/nova/latest/user/metadata.html#openstack-format-metadata
1: https://review.opendev.org/#/c/316395/
2: https://github.com/openstack/nova/blob/261de76104ca67bed3ea6cdbcaaab0e44030f1e2/nova/virt/netutils.py#L282-L309
3: https://github.com/canonical/cloud-init/blob/07104504ab5b30efd2d1f7a8c36effe18b8e5fe0/cloudinit/sources/helpers/openstack.py#L598-L609