Network issue when extending Airport Extreme

This sounds as if the Express was configured to "Extend a wireless network", which is not the correct setting when the Express connects using Ethernet.

In other words, if you configure the Express to "Extend a wireless network" it is expecting a wireless connection from the main router. When the Express sees an Ethernet connection, this creates a huge feedback loop that effectively crashes the network.

The solution is to reset the Express and set it up correctly. Here's how:

Temporarily, unplug the Ethernet cable from the AirPort Express

With the Express powered on, hold in the reset button for 7-8 seconds and release

Allow a full minute for the Express to restart to a slow, blinking amber light status

Very important.....now connect the Ethernet cable to the WAN "O" Ethernet port on the Express

Click on the WiFi menu at the top of the Mac's screen and look for a listing of New AirPort Base Station.

Just under that listing, click directly on AirPort Express

The next window will indicate that the Express will be configured to extend your network. Enter a device name that you want to call the Express and click Next

Watch the next screen very carefully to confirm that the Express is being set up to extend using Ethernet. This will confirm that the setup utility is picking up the Ethernet connection between the AirPorts and the correct settings will be applied to the Express.

In the example below, you can see that an AirPort Express is being set up to extend the network from an AirPort Extreme. You will see your devices displayed on your screen

When you see the message of Setup Complete, click Done.....and you are done.

If you are curious and want to see what setting was actually applied to the Express to extend the network correctly, you can use AirPort Utility to go in and look at the setting for the wireless Network Mode. You may be surprised when you see the actual setting that has been applied to the Express. Hint.....the Express is not set up to "Extend a wireless network" as you might have thought.

You don't have to worry about what setting to apply if you use the setup "wizard". The wizard will automatically apply the correct settings to the Express depending on whether the Express is connected using wireless or using Ethernet.

In other words, if you have NOT established the Ethernet connection between the AirPorts.....before....you run the setup utility, then the utility sees a wireless connection and configures the Express that way.

If the Ethernet connection is established.....before.....you run the setup wizard, then the setup utility will configure the Express correctly for using an Ethernet connection.