On a HW routerboard, you can set the onboard serial baud rate (usually for serial0) with system routerboard settings set baud-rate=<rate>. Another way seems to be to press s during the boot process and select it that way. CHR shows only routerboard=no (for good reason; it isn’t one), but this precludes you from using either boot-delay or the direct baud-rate settings.
The only remaining option appears to be to set the rate from /port. Attempts to change the rate from the line itself yield:
[admin@gns3-test] > port set [ find where name=serial0 ] baud-rate=1000000
failure: Cannot change port while in use!
I’ve also tried to coerce it to happen via /system scheduler firing an on-boot script to set the rate using /port, but on-boot still isn’t early enough to catch it before the line is in use.
Is there a way to force RouterOS to relinquish the serial line (perhaps from a telnet/SSH session) or otherwise get around the catch-22? Any advice appreciated.