AirPort Extreme and TM issue, "looking for backup disc..."

No guarantees, but you would want to make sure that the IPv6 setting for the downstairs AirPort Extreme....the one in Bridge Mode......is set to Link Local Only.

Link Local Only should be set by default when an AirPort is set up in Bridge Mode, but Apple never fixed this bug.

AirPort Utility > Select AirPort > Edit > Internet Tab > Internet Options > Configure IPv6 = Link Local Only > Save > Update

Restart the entire network, power up the "main" AirPort before you power up the "bridge mode" AirPort.

Thanks for that.. really good screenshots.

Big Sur is a bit of a pain when it comes to networking Time Machine backups.

I don't have a computer new enough to run it.. so this comes from a long history of Apple Airport issues and work arounds.

Currently running into an issue with my AirPort Extreme base station loosing connection for TM to backup. Once this happens I can't connect to the AE via AirPort Utility, even when prompted for a pw.

You have a domain set.. this can be an issue.

Open your wireless in network preferences and check what domain is set for search.

You might like to give me a screenshot of that as well.

Here is mine.. I have set domain to local in my main Asus router deliberately as Airports using bonjour are expecting to use local. Just klick the current domain box.. then click the + bottom on the bottom of the box.. type in local.

I can add your domain.. since I already have local.

You can delete a domain that you have added manually but not the automatic DHCP delivered domain. That should be deleted when you add a manual domain.

This has caused issues with lots of people btw on fibre systems.. on cable it might not be same.

DNS is also important.. so for your laptop which I presume you are carrying around with you.. a setup like mine, google public as main dns and the Airport router as secondary dns could also work well..

These changes should help bonjour to maintain discovery of the Airport.

If you have trouble.. particularly airport utility cannot find the Extreme/s then use the alternative method.

Good example.. even with a PERFECT setup.. you can see the airport express has disappeared. It is complicated setup with join wireless on a different AP. Have not had issues for several days. But by setting a static IP you can reach the device using config other.

Just type in the IP address. And password.. click OK.

Lo and Behold

It is alive.. after all.

If the fix of the domain still does not work.. the next step is to force the connection to IP addresses.

It is easy to mount the disk in Finder on your computer by using IP.

In Finder use Go, Connect to Server.

Type in AFP://10.0.1.1 and the password and you should be able to mount both the disk in Finder.. once there Time Machine can generally find it.

In Time Machine preference you will need to add this/these disks to the available Time Machine disks and remove the existing one/s.. IF you cannot fix it via the domain reconfiguration in previous post.

TM will then advise it will begin a new backup but actually discover the existing one and then check it and just add the next incremental. I

Is MonicaUpstairs your main router or do you have a different 3rd party router? This is crucial part of the story.

Which Airport has the hard disk plugged into it for backups?

What OS is your computer running?

I can provide screenshots of each device and current AE setups and network settings. Let me know what you need to see.

Yes.. at least the summary when you click the icon of each Airport will help loads.. then I can show you specific settings. Also your main router info might be needed but mostly I can see it from your Airports.

You don't want to have two AirPorts both trying to handle IPv6 assignments, just as you don't want to have two AirPorts both trying to hand out regular IP addresses on a network.

Link Local Only is the correct setting for any AirPort that is configured to operate in Bridge Mode.

I have never had issues with the "downstairs" becoming unreachable, only the "upstairs" once TM started running backups.

That is really weird.. once you start using IP it should work.

Let me suggest you go a bit deeper and open terminal and use ping command.

Ping the router on 10.0.1.1

If it fails to respond make sure your computer is set correctly and getting an IP 10.0.1.x

The actual computer does use IPv6 to discover the router on the network.. so as Bob says the downstairs should be set to link-local only.

I would add a public IPv4 address to the DNS.