CRS multiple master-port with isolated ports

Hi and happy new year

I have a problem with CRS switch . Lets assume my topology is like http://wiki.mikrotik.com/wiki/Manual:CRS_examples#Port_Level_Isolation with an addition . The ports 15,16,17,18,19,20 should have master port 12 and they should also be isolated .

Actually i just want port isolation like port 5 and port 6 in wiki .

It looks simple but i have tried many ways with no success .

Thanks for help