Port Roles in RSTP

Port Role

Function

Root Port

Same as STP's Root Port (Port with least Cost to reach the RB. Or else Least BID, Least Port Priority, Least Port Number)

Alternate Port

Backup Root port if Root Port fails (same as Blocking Port (in discarding State))

Designated Port

Same as STP's Designated Port (Port with lowest Cost to reach the RB. Or else Lowest BID, Lowest Port Priority, Lowest Port Number)

Backup Port

Backup Designated port if Designated Port fails (in discarding State)

Disabled Port

Same as STP's Port (Administratively disabled)

Edge Port

Ports connected to End devices (where STP is disabled generally)

Blocked ports are marked as Alternate Ports which are basically Backup Root Port.
If Root Port fails, this Alternate port becomes the RP immediately (generally in 1 sec) skipping the Listening & the Learning State.
Backup Port:
The backup port role is only needed in the designs that are little unlikely today (E.g. A Switch connected to a HUB with 2 ports (in same Collision Domain) & HUB is connected to an End device). One from amongst these 2 ports will be Designated port and another port will be blocked marked as a backup port.

Reference:

Last updated 4 years ago