# Set custom MAC on the first bridge (numbers=0)
/interface bridge set admin-mac=6C:3B:6B:18:B5:ED auto-mac=no numbers=0
# Set custom MAC on a specific bridge named bridge3
/interface bridge set admin-mac=6C:3B:6B:18:B5:ED auto-mac=no [ find name=bridge3 ]
# Set custom MAC on the first ethernet port, no matter what it is currently named
/interface ethernet set mac-address=6C:3B:6B:05:B1:A9 [ find default-name=ether1 ]