At the center of RSTP – specifically as an improvement over STP (802.1d) – are the roles that are assigned to the ports. There are four port roles:
Root port The port closest to the root bridge
Designated port The corresponding port on the remote bridge of the local root port
Alternate port An alternate route to the root
Backup port An alternate route to the network segment
The RSTP algorithm usually makes it possible to change port roles rapidly through its fast topology change propagation mechanism. For example, a port in the blocking state can be assigned the role of alternate port. When the backbone of the network fails, the port can rapidly be changed to forwarding.
Whereas the STA passively waited for the network to converge before turning a port into the forwarding state, RSTP actively confirms that a port can safely transition to forwarding without relying on any specific, programmed timer configuration.
