K3CAN

So, while surfing the web and reading up on how I wanted to configure my new BananaPi R3, I encountered a sudden issue with my network connection:

It seemed like I could reach the internet, but I suddenly lost access to my home network services. I tried to SSH into my webserver and was met with a "Permission denied" error.  Since I had earlier attached an additional USB NIC to connect to the BPI, I thought that perhaps the laptop had gotten the interfaces confused and was no longer tagging packets with the correct VLAN ID for my home network. Most of my servers are configured to refuse any requests that don't come from the management VLAN, so this explanation made sense. After poking around in the network settings, all of the VLAN settings appeared correct, but I did notice that the link was negotiated at 100 mbs, instead of the usual 1000. I tired to reconfigure my network settings, manually setting the link to 1000 mbs, resetting interfaces, changing network priorities, etc.  I then tired the classic "reboot and pray" technique, only to find that my wired network connect was down entirely. I wasn't receiving an IP from the DHCP server and the laptop kept reporting that the interface was UP, then DOWN, then UP again, then DOWN again.

Now I started to think that perhaps the issue was hardware related. My usual NIC is built into the laptop's dock, so I thought I would try power cycling the dock itself. This didn't seem to have any effect, besides screwing up my monitors placement and orientation. My next thought was that there might be an issue with the patch cable. "Fast Ethernet" (100mbs) can theoretically function on a damaged cable, so that might explain the lower link speed, and if the damage is causing an intermittent issue, that could also explain the up/down/up/down behavior.  

Being the smart homelabber that I am, I disconnected both sides of the cable and connected my ethernet cable tester. All 8 wires showed proper continuity, though, suggesting that the cable was fine.  When I plugged the cable back in, however, I noticed that I was suddenly getting the normal 1gbe again, but the link was still going up and down. This lead me to the conclusion that the cable likely was the issue, despite it passing the cable test.  I tried replacing the cable entirely with a different one, and found that I now had a stable, 1gbe connection, an IP addresses, and I could now access my network like usual.

Looking back, I think replacing the cable should have been troubleshooting step 1 or 2.

Also, in retrospect, there were some clues that might have let me fix the issue before this point, if I had only put the pieces together. I had noticed another day that the link speed had dropped to 100mbs, but it seemed to correct itself, so I ignored it instead of investigating. While working, I found that Zoom and other applications had started to report my internet connection as "unstable" and that a "speedtest" showed that my internet bandwidth to be drastically lower than it used to be. I assumed this was due to my ISP just being unreliable, since reduced speeds and entire outages are not unusual where I live. 

In hindsight, I think these were all indications that there was a level 1 issue in my network. In the future, I'll have to remember to not over-think things, and maybe just try the simplest solutions first.