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Created attachment 1616113 [details]
System logs with NM trace enabled

Description of problem:

Cannot set `802-3-ethernet.mtu` lower than 1280, the network manager
will still activate the profile with 1280 MTU.

Version-Release number of selected component (if applicable):
NetworkManager-1.20.0-3.el8.x86_64

How reproducible:
100%

Steps to Reproduce:

#!/bin/bash
nmcli c del veth1
ip link del veth1
nmcli c del veth1

ip link add veth1 type veth peer name veth1.ep
ip link set veth1 up
ip link set veth1.ep up
nmcli device set veth1 managed yes

nmcli c add  type ethernet ifname veth1 \
    connection.id veth1 \
    ipv4.method disabled \
    802-3-ethernet.mtu 1200 \
    ipv6.method disabled
nmcli c up veth1
ip link show veth1

Actual results:

The MTU of veth1 show as 1280.

Expected results:

The MTU of veth1 show as 1200.

Additional info:

Added a couple of commits to fix this to merge request

https://gitlab.freedesktop.org/NetworkManager/NetworkManager/merge_requests/273

Fixed on master by:

https://gitlab.freedesktop.org/NetworkManager/NetworkManager/commit/5f284e1574e0c3ab319b504671d263ce89ca12f9
https://gitlab.freedesktop.org/NetworkManager/NetworkManager/commit/353c7c95c1c664c024c4267e67ed2d4b57b44437

This problem does not exists in NetworkManager-1.12.6-5.fc29.x86_64 of Fedora 29.

Regression it is.

RHV customers have the capability to set the MTU lower than 1280, so it is a regression if we do not fix it for RHEL 8.1.z
Could you please backport it?

Per comment c#5, requesting a z stream update for this. Will be required for RHV to pick up this change for their 4.4 release.

Anita, Can you please provide z stream+?

Thanks!
Sushil

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-2020:1847