My mother-board is ASUS AM1I-B/K30BD, with Realtek RTL8169 on board, Intel I350-T4 on the Pcie 3.0x4 slot.
1.Plug the cable between on-board port and modem.
The Realtek driver included in the Openwrt install package. but Intel's doesn't. So I simply setup the WAN(eth0:RTL8169) DHCP, and download the kmod-igb package(opkg install kmod-igb). The mod installs very successfully, system can identify I350-T4 now.
2.Plug the cable between test computer and I350-T4 Port0.
Setup the LAN(eth1:I350-T4) and WAN(eth0:RTL8169), and execute /etc/init.d/network reload. This moment both card run normally with no errors, through the test computer to connect to the Internet is ok either.
3.Reboot the Openwrt
After reboot, when loading drivers, only eth0(now is I350-T4) becomes ready, RTL8169 can't even read by system.
The system tries to load the R8168 module instead of the R8169 and the solution I came up with as to add the 8168 to the blacklist to prevent it from loading the incorrect "driver" and bring the interface up. This in Linux but, should also work in OWRT.
It is kmod-r8169. The RTL8169 is an old chip with a parallel PCI interface. Most likely your MB has an RTL8111 chip, which uses the same driver. There is also a firmware blob associated: r8169-firmware.
The r8169 kernel module covers over 60 different chipsets/ chipset generations by now, some needing a firmware, some optionally needing one (to fix silicon bugs, not strictly necessary under all circumstances), many not needing any. There is a wide variety between these, ranging from PCI to PCIe and 100 MBit/s up to 1000BASE-T and even 2.5GBASE-T.