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Reapplying the MTU does (most of the time) not work:

  nmcli device modify "$DEVICE" ethernet.mtu $((1400 + RANDOM % 100))


I think this is due to https://cgit.freedesktop.org/NetworkManager/NetworkManager/tree/src/devices/nm-device.c?id=17adf58d5dc5d8253f2322e92b96bd176476b6f3#n9074 and https://cgit.freedesktop.org/NetworkManager/NetworkManager/commit/?id=2f8917237fdfb3e4da80989131cc1880013b9e92 (from 1.12.0)

https://gitlab.freedesktop.org/NetworkManager/NetworkManager/merge_requests/127

Fixed: https://gitlab.freedesktop.org/NetworkManager/NetworkManager/commit/4ed72fa658c03790700ba9084e9328fe38afdee9

MTU check added to @device_reapply_all test.
https://gitlab.freedesktop.org/NetworkManager/NetworkManager-ci/commit/ffc2d22df32cca9137bef4b224f653ca2a5ac9a4

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory, and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

https://access.redhat.com/errata/RHBA-2019:2302