To save some of you a lot of trouble (i was searching the forums regarding multicast and found no definite answers to it) i'll post it here:
DIR-655 A3 with firmware 1.21EU in my case failed to forward multicast packets (IPTV in my case) to LAN or wireless from WAN interface (provider has its vdsl2 modem setup in such a way that on all its ports multicast is being broadcasted and it has limit of two active devices on modem). My goal was to have setup like that VDSL2 modem -> dir-655-> LAN pcs and wifi laptops and all LAN and wifi devices IPTV via VLC. To cut long story short i've found an older firmware on taiwanese site (http://www.dlinktw.com.tw/temp/dk2Support_Firmware/632/DIR655A3_FW111EAB08.bin) which after downgrading from 1.21 fixed IPTV broadcasting and everything is A OK.