Airport Express bringing down the entire network?

I got a used Airport Express (2nd gen, the one that looks like a white Apple TV) to extend the wifi range of my Airport Extreme (the tall white monolith looking thing).*

Express rear view for reference:

I ran ethernet from the Extreme to the Express (which is the Apple-recommended method) and when I plug it in to the WAN port, it brings the entire network down… both Extreme and Express are inaccessible.

When I disconnect the ethernet cable, and the Express is connecting to the Extreme via wifi, the Express works to successfully extend wifi coverage. BUT I'd rather have it hardwired together (again, Apple-recommended, and a bunch of other reasons).

What's going on here? How do I get this work?

Here's what I know:

• I actually have two Expresses (only connecting one though) and both behave exactly the same way.
• I know the ethernet cable and its connection to the Extreme is working, because I had another device that used that cable/port and it was working fine (Netgear Arlo Pro Hub).
• From the image above, it actually looks like the WAN and LAN ports are interchangeable(?). So I've tried on both ports. No difference.
• All devices have been upgraded to the latest firmware.

*Yeah, I know: this is ancient tech. But up until this point my home wifi network has been rock solid. I got the pair of used Expresses for $30 total… so why not give this a shot?

Solution 1:

You might be having a situation in which you have a conflict because both devices are trying to act as DHCP servers, and perhaps serving addresses in the same range.

Make sure your new Airport Express is configured in "Bridge" mode before connecting it to your network. I don't have an Airport here, and don't remember exactly what Apple calls the WiFi Router modes. You want a mode in which the new Airport Express does NOT do DHCP or NAT routing, and just passes packets to and from your main Airport router.

Get it configured in Airport Utility before trying to connect it to your network. You can temporarily connect it to your computer with an Ethernet cable. (Use the Airport's LAN port for this.)

Then, you should connect an Ethernet cable from a LAN port of your main Airport to the WAN port of your new Airport Express.

Solution 2:

In this situation, where you're connecting your Express to your Extreme via ethernet (and not wifi) you actually want to choose Create a wireless network for the Network Mode, and not Extend a wireless network:

To me, this is completely counter intuitive. I am extending my wireless network and not creating one... a network already exists?! But, well, there you have it.

Anyway, after discovering this solution via trial and error, I Googled for the difference between these two modes and found this:

Create a wireless network is used only when the Airports have an Ethernet connection. If you are using the setup wizard.....the setting will read Extend using Ethernet during the setup. If you go back in using AirPort Utility to check the Network Mode setting.....you will see that it is really Create a wireless network.

Extend a wireless network is used only if an AirPort is connecting to another AirPort using wireless.......not Ethernet.

Every one of the Airports on your network must be configured to Create a wireless network.

If you do connect an Ethernet cable to an AirPort that is already set up to Extend a wireless network......that will create a massive feedback loop that will crash the network until you remove the Ethernet connection to the device.

So apparently, the combination of extending the wifi network and using an ethernet cable will create a feedback loop that crashes the network... which is exactly what I was experiencing.

And now I know.