Command
Spanning-Tree Cost
Use
This command modifies the interface cost used in calculating the root port for spanning-tree on a switch. Lower is better.
Syntax
Switch(config-if)#spanning-tree cost <cost>
Example
In the below example we will configure SW2 to use Fa0/24 as the root port for all VLANs.
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 cost of 1.
SW2(config)#int fa0/24 SW2(config-if)#spanning cost 1
After we change Fa0/24 to have a lower cost, it becomes the root port on SW2
SW2(config-if)#do show spanning vlan 10 VLAN0010 Spanning tree enabled protocol ieee Root ID Priority 8202 Address 0019.060c.4f80 Cost 1 Port 26 (FastEthernet0/24) 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 Altn BLK 19 128.25 P2p Fa0/24 Root FWD 1 128.26 P2p