Spanning-Tree Port-Priority Command on CISCO Router/Switch

Command

Spanning-Tree Port-Priority

Use

Sets the switches preference for the alternate paths to use if a loop is detected.

Syntax

Switch(config)#spanning-tree port-priority <number>

Example

In the below example we will configure SW2 to prefer to use Fa0/24 if a loop is detected in the network.

SW2(config)#do show spanning vlan 10

VLAN0010
Spanning tree enabled protocol ieee
Root ID Priority 8202
Address 0019.060c.4f80
Cost 19
Port 25 (FastEthernet0/23)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Bridge ID Priority 16394 (priority 16384 sys-id-ext 10)
Address 0019.060c.cb00
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 15

Interface Role Sts Cost Prio.Nbr Type
—————- —- — ——— ——– ——————————–
Fa0/19 Desg LRN 19 128.21 P2p
Fa0/20 Desg LRN 19 128.22 P2p
Fa0/21 Desg FWD 19 128.23 P2p
Fa0/22 Desg LRN 19 128.24 P2p
Fa0/23 Root LRN 19 128.25 P2p 
Fa0/24 Altn BLK 19 128.26 P2p

Next, we configure Fa0/24 for VLAN 10 to have a port-priority of 16.

SW2(config)#int fa0/24
SW2(config-if)#spanning port-priority 16

Show spanning vlan 10 reflects the change.

SW2(config-if)#do show spanning vlan 10

VLAN0010
Spanning tree enabled protocol ieee
Root ID Priority 8202
Address 0019.060c.4f80
Cost 19
Port 25 (FastEthernet0/23)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Bridge ID Priority 16394 (priority 16384 sys-id-ext 10)
Address 0019.060c.cb00
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300

Interface Role Sts Cost Prio.Nbr Type
—————- —- — ——— ——– ——————————–
Fa0/19 Desg FWD 19 128.21 P2p
Fa0/20 Desg FWD 19 128.22 P2p
Fa0/21 Desg FWD 19 128.23 P2p
Fa0/22 Desg FWD 19 128.24 P2p
Fa0/23 Root FWD 19 128.25 P2p
Fa0/24 Altn BLK 19 16.26 P2p

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