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ASUS RT-AX89X > 10G base -T as WAN not working, ISP works fine on 1Gb WAN Port (UK - BT Fibre 900)

Level 7

Hi All,

As per  the title really.

I am having issues setting up the 10G bast-T port as a WAN interface.
When I plug the cable in, I get a random 10.xx IP address, this is not the IP address expected from the ISP.
BT usually have 86.xx,  89.xx, 109.xx etc

I am going to WAN > Dual WAN
Leaving "Enable Dual WAN" = OFF but setting the "Primary WAN" to 10G bast-T 
Router then restarts, I swap over the cable and its like DHCP is not working on that port.

Has anyone experienced this issue, or know how to troubleshoot / resolve further.

Thank you



Level 14

Hard reset the router.  Plug the ONT/MOCA/modem directly to the 10 Gb port.  Start up the router and try to set it up.

Level 12

It might be best if you rebooted the gateway/modem, after you had everything set.  Even rebooting both might help.

You are certain the port is usable as a WAN?

Maximus Z790 Hero,
Intel i9-13900k
Intel BE200

Level 14

(I'm assuming you are connecting the RT-AX89X directly to the Openreach ONT.  If you are connecting it to an ISP router (e.g. a BT SmartHub), that's a different and additional set of complexity.)

I could be missing something, but you probably won't gain anything by using the 10G port for WAN.  1G Ethernet is full duplex, so can do 1000Mbit RX and 1000Mbit TX simultaneously, which seems like it would be sufficient for the BT Fibre 900 service (900Mbit RX and 110Mbit TX).  A 10G port connected to a 1G port shouldn't have a significant performance benefit over just 1G to 1G; at the Ethernet level, both scenarios are identical (assuming the auto-negotiation works correctly).

If the default port setup for the router is sufficient for the service, I would stick with that, personally.  ASUSWRT is pretty good as far as retail consumer router firmware goes, but it can get a bit feisty in less common configurations.  You may well only have a 1G port on the Openreach ONT anyway, as it was only last year that they were talking about moving to a newer ONT with a 2.5G port (the original / previous Openreach FTTP ONTs were all 1G Ethernet, as far as I know).

Rebooting the ONT is certainly a good idea when connecting it to something else (even just a different port on the same router), when you encounter problems like this.

Hi All,
Thank you for all the information.

I am up and running on the WAN port, as you rightly said the ONT is only 1G, I was just curios to see if the 10G Port offered extra beifits with the addional bandwidth availible.

Still not too sure why the WAN DHCP failes even after a power cycle on the BT ONT but for now its all ok 🙂