Just had an interesting situation with a Mikrotik CRS328-24P-4S+RM. Customer called and stated that their network was down. We went on-site to find that someone had plugged a network cable into the console port of the switch that was running to another switch into the network port. The switch will hang on boot if it is plugged in this fashion.
We did some testing at the shop and found that if the switch is already booted we can plug a network cable from one switch network port into the console port of the Mikrotik and it will continue to work until we power cycle the switch. The switch hangs after the power cycle. If we unplug the cable from the console port in around 30 seconds the switch reboots and goes back to functioning normal. I tried disabling the console port as well as removing it completely but the issue happens with the cable plugged in no matter what.
This managed customer just so happens to be an hour and thirty minute drive one way to find out that someone plugged in a network cable incorrectly.
“Having text coming out of the serial port to the connected device might confuse your attached device. Furthermore in standard config you can enter RouterBOOT menu by pressing ANY key. So if your serial device sends any character to the serial port of your RouterBoard during boot time, the RouterBoard will enter the RouterBOOT menu and will NOT boot RouterOS unless you manually intervent!”
If you navigate to System → Routerboard → Settings in winbox I believe you can disable the pre-routerOS serial interface by selecting off in the Baud Rate dropdown menu. Checking silent boot would be the other option to try.
I am not 100% sure because I didn’t test it, but there is protected-routerboot option. This is extremely dangerous as it disables both netinstall and console access. If your device malfunctions and you can’t log in via network, you will have little chances to restore it.
Due to that, I would also suggest plug and sticker. If you are brave, even hot-melt-adhesive would do the job, but you must be careful and avoid touching wires - if it gets stuck to wires, it might rip them out once you try to open it.
It is definitely not a bug. Ethernet communication unfortunately appears to serial console as bashing to the keyboard without any sense. And during boot, process stops if there is any input - that is intentional feature.
Once system is booted, console input does not do anything except possible “warning, incorrect login…” log entries.
So disabling the baud rate on the serial port takes care of the issue. Knowing that I could possibly run into an issue in the future and need that port I am going to put a sticker over every one of them so the end user doesn’t get any bright ideas…
