Realtek RTL 8125
The RTL 8125 is a 2.5 GiB Ethernet network chip for 2.5 Gigabit networking over standard CAT5e/CAT6 wiring. Cards with this chip have a standard RJ45 port, SFP modules are not required or supported. The chip comes in several slightly different revisions (RTL8125A, RTL 8125B, etc).
Cards with a the RTL 8125 are very attractive because they are so cheap; single-port 2.5 Gigabit PCIe RJ45 cards can be had for $15. Re-purposing an old computer to be a 2.5 Gigabit switch can be as cheap as $60 (+the old computer if you do not have one) which is relatively cheap compared to the outrageously priced switches with 2.5 Gigabit Ethernet that are typically limited to two 2.5 Gigabit RJ45 ports.
The Linux kernel supports network cards based on the RTL 8125B chip as part of general Realtek support in the
r8169 kernel module ( ▸ ▸ ▸ ).
Linux support for the RTL 8125 is relatively new and newer revisions like RTL 8125B require a very new kernel. Linux 5.4.x is the first long term supported Linux kernel with support for the RTL 8125 chip. It supports the RTL 8125A, but it does not support RTL8125B. Support for RTL8125B was not introduced until Linux 5.9.
|Kernel||RTL 8125||RTL 8125A||RTL 8125B|
Do note that you can't use any RTL 8125 chip with a 4.x kernel, and you should also note that the latest RTL8125B revision requires Linux 5.9.x.
There are plenty of very cheap 2.5 Gigabit cards based on the RTL 8125 available on sites like AliExpress.
|DIE WU TXA073||RTL8125A||2.0 x1||$15-$25|
|DIE WU TXA092||RTL8125B||2.0 x1||$15-$25|
Variants of the RTL8125 can also be found on a lot of motherboards (MSI B550 Tomahawk, etc).