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Created attachment 1791963 [details]
Reproduce script

Description of problem:

When using `nmcli c  mo br0 remove 802-3-ethernet` to remove MTU/etc settings,
the MTU actually decreased 1.

Version-Release number of selected component (if applicable):
NetworkManager-1.30.0-7.el8
NetworkManager-1.32.0-0.5.el8


How reproducible:
100%

Steps to Reproduce:
1. sudo ./mtu_bug.sh
2.
3.

Actual results:

The MTU of br0 changed to 1399

Expected results:

The MTU of br0 revert back to default 1500.

Additional info:

Created attachment 1791975 [details]
System logs with NM trace enabled

Hi Beniamino,

Please take a quick a investigation to check whether this is bug is trivial or the tip of the iceberg.

Thanks!

Hi, this is the result of:

https://gitlab.freedesktop.org/NetworkManager/NetworkManager/-/commit/e23798a5e5e8d7eb615f0e8b305fc20b8b645c66

The problem is that NM stores (MTU - 1) as the initial MTU and then it resets the MTU to the stored value on deactivation. A fix is should be not too difficult.

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 (NetworkManager bug fix and enhancement update), 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/RHEA-2022:1985