The roles were in fact introduced by RSTP. Because it was also very convenient with regular STP, we added them to our implementation of STP. However, older software are just showing the information defined in STP at that time.
A forwarding port is indeed designated or root. In order to make a difference between the two, you need to check what is the designated bridge ID. If this is the local bridge, the port is designated. If it's a different bridge, it's a root port.
Another simple way: you also get the root port for the vlan in the show spantree. There is only a maximum of one root port on a bridge, so if your forwarding port is not the root port, it is then designated.
Note that STP does not make any difference between backup and alternate port either. For this, you need again to look for the designated bridge ID on this port. If it is the bridge itself, this is a backup port, else, an alternate port (this is useful for uplinkfast, only alternate port can do fast transition).
Regards,
Francois
