I am just a bit confused about designated ports in STP, i know how to determine them and the confusion is about the say " designated port is the only port in charge to forward traffic on a segment ". my question is : in many cases we have segments or links that a root port on one end and a designated port on the other end here the two ports are forwarding traffic that does not agree with the say " designated port is the only port in charge to forward traffic on a segment ".
In short and simplified, a designated port is a forwarding port that leads away from the root bridge (from the priority perspective).
The root port is the best port leading towards the root bridge. A root port on one bridge needs to be designated at the other end bridge for the tree to work. The algorithm takes care of this by blocking only alternate ports leading towards the root bridge.
You might want to check the reference IEEE 802.1Q 13.12 Port role assignments.