MAAS picks up information at commissioning time about the system. This
would include the NIC driver, which could just as well be used as an
additional matching criterion in netplan YAML.

There's a limit to how much netplan can and should try to figure out
what one means: we're supposed to do *translation* of configuration --
sure, we could check the running system at the time to try to make the
config more precise, but that only helps *iff* the user wasn't actually
precisely requesting what they wanted. Anything we add to what's written
in the actual netplan YAML needs to be carefully weighed against whether
we risk actively going against what the user has written. The user knows
best what their network is like.

This is why I'm strongly suggesting that in addition to what I'm adding
in netplan, changes are made to MAAS to ensure the config is as precise
as it possibly could be, while also not including default values on
optional fields without the admin making a conscious decision on adding
these values (ie. default MAC for a bond). MAAS knows what the MAC for a
physical device is, what its name should be, and even down to the
driver. Sure, any of those could change, but that's not different from
any other environment: devices fail, hardware needs replacement. The MAC
would have changed anyway. At least with MAAS you can re-commission and
re-deploy systems easily when that happens.

I've added
https://github.com/CanonicalLtd/netplan/commit/a27122bc8d8e066b1a90a7fd8d65342e8b906a8e
to netplan work mitigate the current issue. This shouldn't be considered
an end-all solution -- I still think other pieces of the network stack
need changing.

It's as much as I am comfortable adding to netplan to mitigate the
problem. There isn't enough information coming from the YAML to figure
out more details about the interface, nor a non-hackish way to retrieve
the information from the system when networkd will do the matching
later.

** Changed in: netplan
       Status: New => Fix Committed

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https://bugs.launchpad.net/bugs/1804861

Title:
  MTU size defined on /etc/netplan/50-cloud-init.yaml not applied

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