Spanning-Tree Cost Command on CISCO Router/Switch

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

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