Network connection lost ('carrier-changed')

Solution 1:

After removing other factors, such as the switch, the problem continued.

I asked one of the system administrators of our organisation's network and he seems to think that such an error message means that there is a physical problem with the network.

For example, with the switch gone, this means:

1. cable from the computer to the port
2. port itself
3. even the cabling behind the port, within the wall

In any case, he said it is "definitely" not related to the OS but was unable to decipher the error message piece-by-piece. So, this is where the problem stands and I'll be looking at the physical connection to the computer instead of Ubuntu.

Solution 2:

In my case carrier-changed coincides with the router disconnecting/reconnecting with the internet (see the logs in your router). The cause may be a not well connected cable, but there are several other possibilities: cable provider initiates a reconnect; a router may have problems (firmware; data backup loaded from an other router where the data somehow confuses this router ...).

However, the computer should successfully reconnect to the router itself when the router has its uplink working again. In my case the router needed 12 minutes (!) with several "cable internet synchronising (training)" attempts before establishing a stable uplink. Maybe the cable provider installs several firmware updates remotely ( I can't even see what firmware is running at the router (fritzbox 6490), the cable provider has full control - annoying!) - but this is just speculation.

I forced a reconnect of the router myself which took only 7 secs. The computer has had no problem reconnecting in this case. I assume this is because of the short time frame.

Question remains: how to bring the computer to self heal a carrier-change even if this change takes a lot of time.