[Bug 1717471] Re: networkd does not accept / set advertised mtu

[Mathieu Trudel-Lapierre]

Verification-done for xenial 0.32~16.04.3, and zesty 0.32~17.04.1:

UseMTU=yes is set on the interfaces for which a MTU is set, and the MTU
coming from DHCP is applied correctly when running 'netplan apply'.

** Tags removed: verification-needed-xenial verification-needed-zesty
** Tags added: verification-done-xenial verification-done-zesty

** Tags removed: verification-needed

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Title:
  networkd does not accept / set advertised mtu

Status in nplan package in Ubuntu:
  Fix Released
Status in systemd package in Ubuntu:
  Fix Released
Status in nplan source package in Xenial:
  Fix Committed
Status in systemd source package in Xenial:
  New
Status in nplan source package in Zesty:
  Fix Committed

Bug description:
  [Impact]
  Hosts may require a specific MTU to be set as passed by DHCP options. We should honor these settings to ensure proper communication of the host with the rest of the network.

  [Test case]
  1) Run netplan on a system that should receive MTU settings from DHCP.
  2) Validate that the MTU configuration provided by DHCP is applied to the right interface.

  [Regression Potential]
  If DHCP settings specify an invalid MTU setting which is currently being ignored and letting the systems communicate correctly with the network, then these systems would regress.

  ---

  <wgrant> Right, so as far as I can tell the neutron-api on lcy01 is
  configured to advertise an instance interface MTU of 1400, though
  that's lower than it has to be, and the new DHCP setup with networkd
  just doesn't respect that DHCP option.

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