Setting up OpenWrt on the DSL-AC68U for 1 Gig speeds

Adam Jones

Personally

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22 December 202422 December 2024

With default settings, OpenWrt will only reach 360Mbps download with a DSL-AC68U. This guide will explain how you can set up this device with OpenWrt, and achieve speeds of over 900Mbps!

Note: At time of writing, OpenWrt does not support WiFi or ADSL/VDSL on this device. This is fine for me, as I’m just using it as an ethernet router. If you need these capabilities, consider using the stock firmware or Asuswrt-Merlin (gnuton's version).

Flashing

Warning: This should be pretty safe and Asus makes it pretty easy to recover a router from a weird state. That said, I take no responsibility if you break your router.

You will need:

• 1x DSL-AC68U
• 1x Ethernet cable
• 1x Computer with ethernet port

Follow the guide from the OpenWrt wiki for the RT-AC68U. In short:

1. Turn off the device
2. Press and hold the reset button
3. Turn on the device
4. Wait until the power LED blinks slowly (in about 10 seconds). Then release the reset button.
5. On your computer, download the latest¹

   While I recommend using the latest firmware, one that definitely worked for me was 23.05.5

   ‘Factory image’ linked on the wiki
6. Connect your computer with an ethernet cable to any LAN port (you may need to set your computer to have a static IP)
7. Visit 192.168.1.1 in your browser, which should show the CFE miniWeb Server
   • Don’t see anything? Do an NVRAM reset, go back to step 1.
8. Upload the factory image you downloaded and wait for the upgrade (in about 3 minutes)
9. Login to OpenWrt at 192.168.1.1 in your browser (username: root, password: leave blank)
10. Do an NVRAM reset.

Break something along the way? Do an NVRAM reset, then start again from step 1.

To go back to the stock (original) firmware, download it here (unzip it to get the .trx file) and follow the steps above but upload the stock firmware at step 8.

Configuration

Configure OpenWrt as normal. If you’re new, you probably want to:

• Open Luci (the web portal for configuring OpenWrt) at 192.168.1.1
• Set a router password (you’ll be prompted to do this automatically)
• Configure a WAN port (to connect your devices to the internet)
  • [This assumes you want to use the LAN 1 port for WAN. Adjust accordingly if not.]
  • Connect your WAN cable (the one from your internet provider) to LAN 1
  • Connect your computer to any other LAN port
  • In Luci, go to ‘Network > Interfaces’
  • In the ‘Interfaces’ tab (selected by default)
    • Click ‘Edit’ for ‘wan’, and set the device to lan1
    • Click ‘Edit’ for ‘wan6’, and set the device to lan1
  • On the same page in the ‘Devices’ tab
    • Click ‘Configure…’ for br-lan, and in ‘Bridge ports’ remove lan1. Then ‘Save’.
    • Click ‘Unconfigure’ for wan.
  • Click ‘Save & Apply’

If you make bad changes and can’t reach the router, just wait 90 seconds and they’ll be reverted.

Performance tuning

With the configuration above, OpenWrt achieves about 360Mbps download. This might be fine for many use cases, but we can do better. There are two tricks: software flow offloading, and increasing the CPU clock speed. These two can enable us to reach speeds of over 900Mbps!

Software flow offloading

The default firewall implementation may slow down traffic considerably. Enable offloading to speed this up:

1. Open Luci (the web portal for configuring OpenWrt) at 192.168.1.1
2. Go to ‘Network > Firewall’
3. Tick the box for ‘Software flow offloading’, then ‘Save & Apply’

Increasing CPU clock

The OpenWrt firmware we installed was for the RT-AC68U, which has a lower default clock speed. We can increase the clock speed back to what the DSL-AC68U supports without sacrificing stability.

To do this:

1. SSH into the router. On most systems this will be running ssh root@192.168.1.1 in the terminal and using your router password. See the OpenWrt wiki for more details about SSH.
2. On the router, run: nvram set clkfreq=1200,666 && nvram commit && reboot. This will increase the CPU frequency to 1200Mhz, set the RAM frequency to 666Mhz, and reboot the router. You’ll need to wait for a minute for the router to reboot.
   • You can tweak the number even higher if you want, which might eek out a bit more performance. I didn’t bother as I was already getting the max from my external internet connection. If you push it too high, the router might not boot - an NVRAM reset will undo your changes and fix this.

Full benchmarks

Here’s the full table of benchmarks for the DSL-AC68U on OpenWrt, plus a comparison against the stock firmware:

*This is the fastest I have ever been able to get with any router on this connection. So it might be that this is simply the limit of my connection rather than the router at this point.

Appendix: NVRAM reset

1. Turn off the device
2. Press and hold the WPS button
3. Turn on the device
4. Wait until the power LED blinks fast (in about 30 seconds). Then release the WPS button.