Description of problem:

The cluster network MTU change procedure described in [1] is not reflected in the cluster.

• The affected cluster had its MTU modified on Day 1 using the Baremetal UPI installation method with ISO, nmcli and the --copy-network option ([2]).

• The configuration applied via method [2] (Day 1 ISO/nmcli/--copy-network) results in the final network settings being placed in files within /etc/NetworkManager/conf.d/.

• The configuration applied via method [1] (post-installation MTU change) results in the final network settings being placed in files within /etc/NetworkManager/system-connections/.

• We believe the configuration files in /etc/NetworkManager/system-connections/ take precedence over those in /etc/NetworkManager/conf.d/, which prevents the MTU change applied via method [1] from taking effect.

I suggest adding a note or warning to documentation with the following intent:

• Add "This method for changing the MTU does not work for clusters where the MTU was modified on Day 1 using nmcli or the --copy-network option." to document [1].
• Add "If you need to change the cluster network MTU on Day 2, you must not set the MTU" to document [2].

Or, if modifying the MTU on Day 1 using nmcli and --copy-network is fundamentally unsupported, a corresponding note should be added to documentation [2] with the following intent:

• Add "Changing the MTU using nmcli and the --copy-network option is not supported." to document [2].

[1] https://docs.redhat.com/en/documentation/openshift_container_platform/4.19/html/advanced_networking/changing-cluster-network-mtu#nw-cluster-mtu-change_changing-cluster-network-mtu

[2] https://docs.redhat.com/en/documentation/openshift_container_platform/4.19/html-single/installing_on_bare_metal/index#installation-user-infra-machines-advanced_network_installing-bare-metal