Issue #40: Finish documenting Rosewill 2.5 Gbps card.

Author: geerlingguy

_cards_network/rosewill-rc20001-25gbe.md

@@ -2,10 +2,29 @@
 layout: card
 title: "Rosewill RC-20001 2.5GBASE-T PCIe x1 Network Adapter"
 picture: "/images/network-rosewill-rc20001.jpg"
-functionality: "Currently Testing"
+functionality: "Full"
 driver_required: "Yes"
 github_issue: "https://github.com/geerlingguy/raspberry-pi-pcie-devices/issues/40"
 buy_link: https://amzn.to/3nJPXP9
-videos: []
+videos:
+  - https://www.youtube.com/watch?v=wCbQQ5-sjGM
 ---
-I just got this card. See the GitHub issue for more details.
+This adapter works great with the Pi in the built-in 1x PCIe slot on the IO Board. I tried installing the Realtek drivers from their website but had difficulty compiling them.
+
+So instead I recompiled the Pi OS kernel and used menuconfig to select the right driver to build:
+
+```
+Device Drivers
+  > Network device support
+    > Ethernet driver support
+      > Realtek devices
+        > Realtek 8169/8168/8101/8125 ethernet support
+```
+
+After compiling the kernel and moving it to the Pi, I rebooted and the card was identified as `eth1`. It connected fine to my 1 Gbps network, as well as my 10 Gbps network through a [MikroTik switch](https://amzn.to/3mRSs0I) (at 2.5 Gbps).
+
+To get full 2.5 Gbps throughput, I had to enable Jumbo Frames (MTU 9000) using `sudo ip link set dev eth1 mtu 9000` and make sure all the other devices on the network path supported Jumbo Frames.
+
+Otherwise, you are limited to around 1.9 Gbps without an overclock on the Pi, due to IRQ interrupts.
+
+See this blog post for more details: [Testing 2.5 Gbps Ethernet on the Raspberry Pi CM4](https://www.jeffgeerling.com/blog/2020/testing-25-gbps-ethernet-on-raspberry-pi-cm4).