1. See the ethernet block diagram here,
https://espressobin.net/mochabin_wiki/tiki-index.php?page=Block+diagram
2. see also the ports and interfaces photo here,
https://espressobin.net/mochabin_wiki/tiki-index.php?page=Ports+and+Interfaces
(a) Eth0 is 10Gb SFP+ optical WAN port
(b) Eth2 1Gb SFP optical WAN port share with RJ45 both came from the Gb PHY chip.
The PHY is configured to check which of the SFP and RJ45 ports is cable connected then assign it as the WAN port. The SFP port will be selected if both cables are connected.
(c) Eth0 and eth2 are networks bonded in mode-1(active backup) as the same WAN port with eth2 set to bond-primary. This means eth0 will be active only when eth2 doesn’t exist or fails
(d) ETH1, 1Gb LAN0, LAN1, LAN2, LAN3 all come out of Gb eth switch chip
3. As default with ubuntu rootfs, see /etc/network/interfaces
4. The network setup may change with different distribution like Ubuntu, archlinux or OpenWrt
