Can't specify speed and duplex ethernet properties when link auto-negotiation is enabled

Issue

• Can't specify speed and duplex 802-3-ethernet properties when link auto-negotiation is enabled on RHEL7.4

# nmcli con modify eth0 802-3-ethernet.speed 100 802-3-ethernet.duplex full 802-3-ethernet.auto-negotiate on
# grep ETHTOOL_OPTS /etc/sysconfig/network-scripts/ifcfg-eth0
ETHTOOL_OPTS="autoneg on"

• NetworkManager does not allow 802-3-ethernet properties to be modified
• Trying to set any of the 802-3-ethernet properties returns an error that the property cannot be changed.

# nmcli connection show em1 | grep 802-3
connection.type:                        802-3-ethernet
    802-3-ethernet.speed:                   0
    802-3-ethernet.duplex:                  --
    802-3-ethernet.auto-negotiate:          yes

# nmcli connection modify em1 802-3-ethernet.auto-negotiate off
Error: failed to modify 802-3-ethernet.auto-negotiate: the property can't be changed.

# nmcli connection modify em1 802-3-ethernet.speed 100
Error: failed to modify 802-3-ethernet.speed: the property can't be changed.

# nmcli connection modify em1 802-3-ethernet.duplex full
Error: failed to modify 802-3-ethernet.duplex: the property can't be changed.

# ethtool -s em1 autoneg off speed 100 duplex full
# echo $?
0