A backup port is an RSTP port that provides a redundant path to the same network segment as a designated port on the same switch. It remains in the discarding state to prevent a Layer 2 loop. section 15.2.3 section 15.5
Identify a backup port by checking these conditions:
- Two ports on the same switch connect to the same segment or collision domain, such as through a hub. section 15.2.3
- The switch is the designated bridge for that segment, but the port is neither root nor designated. section 15.2.3
- Modern networks rarely produce this role because hubs are uncommon, but it remains relevant for exam questions. section 15.2.3
Recognizing the role in a topology
If SW3 is the designated bridge and has two ports connected to the same segment, SW3 G0/1 becomes a backup port while SW3 G0/0 remains designated. The command output identifies the backup role as “Back” and its state as “BLK.” section 15.2.3
SW1# show spanning-tree . . . Interface Role Sts Cost Prio.Nbr Type ------------------- ---- --- --------- -------- -------------------------------- Gi0/0 Root FWD 4 128.1 Shr #1 Gi0/1 Altn BLK 4 128.2 Shr #2 SW3# show spanning-tree . . . Interface Role Sts Cost Prio.Nbr Type ------------------- ---- --- --------- -------- -------------------------------- Gi0/0 Desg FWD 4 128.1 Shr #3 Gi0/1 Back BLK 4 128.2 Shr #4 #1 SW1 G0/0 is a root port. #2 SW1 G0/1 is an alternate port. #3 SW3 G0/0 is a designated port. #4 SW3 G0/1 is a backup port.
Use the spanning-tree output to distinguish a backup port from an alternate port: the role appears as Back and the state as BLK. section 15.2.3
