Port Roles in RSTP
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) |
Alternate Port:
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:
CCNA 200-301 OCG, Volume 1, Pg. 229-233 - Wendell Odom.
RST Notes
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