OpenWRT hands-on with 2.5gigabit-NICs

Hardware Questions and Recommendations
1

I bought a DIGITUS 4x-Port RTL8125B 2,5 GBits card for my 8gen Intel x86 router/NAS (OpenWRT 24.10.0). (was 68€ at the German Bezos-store)

  • 4x RTL8125B, 4 dedicated NICs (not a single NIC with switch)
  • card is fanless, but gets hot after a while, also in idle (not iron-melting-burning-hot, but still hot)
  • card claims to use 2 lanes of a x4 PCI express, I get inconsistent lspci output, when installed in the GPU pci express port of the intel 8gen board (guess is, my buggy Asrock mainboard is the cause). Placed in the secondary x16 port for now, which is connected via platform controller hub.
  • card has a non-intuitive non-linear mapping of the MAC addresses to the physical ports: 3,4,1,2 (instead of a more intuitive 4,3,2,1 like my previous 4x NIC)

I also bought a TP-Link USB-C 2.5G LAN USB-dongle for 17€ to upgrade an older intel Win11-PC. Ugreen seems to have an USB adapter that looks identical.

  • RTL8156 based
  • dongle gets warm, but not hot
  • dongle also works flawlessly, when attached via USB to my OpenWRT x86

I've tested it with yet another 2023 Win11 client that has a single 2.5gb onboard realtek 8125 and a GL-MT6000 (plain OpenWRT) with its 2x 2.5 gbit ports (which I guess are also 8125)

iperf3 impression:

x86 OpenWRT / Digitus 4xNIC <-> anything 2.5G : ~2,35gigabit/s
Win11 / USB Realtek 8156 <-> anything 2.5G : ~2,35gigabit/s
(even if hopping over 2x 2.5G ports of a MT-6000)

Update Nov-03-2025:

  • I have updated the throughput numbers above, no more slow throughput.
  • Missed that: The USB Realtek adapter is a Realtek 8156, not 8125 as I thought when initially creating this post.
  • The Realtek 2.5G USB dongle no longer sucks in throughput: Win11 24H2/25H2 has a default Realtek LAN driver being active automatically, but that driver claims to be several years old. Don’t use it, it works, but is slow. Little did I know at that time. I’ve now installed a driver directly from the Realtek web site (“Realtek USB FE / GbE / 2.5GbE / Gaming Family Controller”) and now i get 2,35gigabit/s consistent in both directions via iperf3.

Older Impression:

  • the fanless DIGITUS 2.5 gigabit/s 4xNIC PCI express card feels quite hot after a while, even in idle. Adding a fan or larger cooler might not be bad idea in a closed/tight case. Probably this also applies to other 4x 8125 PCIe cards (No comparison to my previous 4x1gbit Broadcom card, which did not even try to get warm).
2

Cheap r8125b PCIe cards from overseas on both ends (kabylake).

iperf3

[ ID] Interval           Transfer     Bitrate         Retr
[  5]   0.00-10.00  sec  2.70 GBytes  2.32 Gbits/sec    0            sender
[  5]   0.00-10.00  sec  2.70 GBytes  2.32 Gbits/sec                  receiver

iperf3 --reverse

[ ID] Interval           Transfer     Bitrate         Retr
[  5]   0.00-10.00  sec  2.70 GBytes  2.32 Gbits/sec    0            sender
[  5]   0.00-10.00  sec  2.69 GBytes  2.31 Gbits/sec                  receiver