Take into account what is written in this whole topic: http://forum.mikrotik.com/t/hex-block-diagram/114819/1
Short version, before 7.1rc5 the mt7621 port/cpu lane layout was broken, in versions before 7.1rc5 you have to use port1 as WAN and ONLY 2 and 4 as LAN (while 2,3,4,5 are bridged).