https://www.raspberrypi.org/forums/viewtopic.php?p=1691385#p1691385
> "There are 3 ethernet PHY power states that you enter based on various conditions. The first is active/transmitting - when there is traffic, the PHY is fully powered and sending/receiving data. The second is auto-GREEN mode, where the PHY enters a transparent energy-efficient mode when there is no active traffic (stops sending idle symbols and powers down a lot of the TX circuitry, and enters a sleep mode on the RX side). This requires a compatible Energy-Efficient Ethernet link partner, but most switches implement this feature by default. The third is with the cable unplugged - the PHY enters "energy detect" idle which will completely powers down TX and most of RX, periodically waking the RX side to check for cable insertion."