Spanning-Tree VLAN Root Primary Command on CISCO Router/Switch

Command

Spanning-Tree VLAN Root Primary

Use

This command sets the switch to become root for a given VLAN. It works by lowering the priority of the switch until it becomes root. Once the switch is root, it will not prevent any other switch from becoming root.

Syntax

Swith(cconfig)#spanning-tree vlan <vlan range> root primary

Example

In the below example we will configure SW1 to become the root for VLAN 10. Currently, we see that SW1 is not root for vlan 10. Notice the priority is currently 32768

SW1(config)#do show spanning vlan 10

VLAN0010
Spanning tree enabled protocol ieee
Root ID Priority 24586
Address 0019.0606.b600
Cost 19
Port 23 (FastEthernet0/23)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Bridge ID Priority 32778 (priority 32768 sys-id-ext 10)
Address 0012.00cb.6c80
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 15

Interface Role Sts Cost Prio.Nbr Type
—————- —- — ——— ——– ——————————–
Fa0/1 Desg FWD 19 128.1 P2p
Fa0/2 Desg FWD 19 128.2 P2p
Fa0/4 Desg FWD 19 128.4 P2p
Fa0/5 Desg FWD 19 128.5 P2p
Fa0/8 Desg FWD 19 128.8 P2p
Fa0/9 Desg FWD 19 128.9 P2p
Fa0/10 Desg FWD 19 128.10 P2p

Interface Role Sts Cost Prio.Nbr Type
—————- —- — ——— ——– ——————————–

Fa0/19 Desg FWD 19 128.19 P2p
Fa0/20 Desg FWD 19 128.20 P2p
Fa0/21 Desg FWD 19 128.21 P2p
Fa0/22 Desg FWD 19 128.22 P2p
Fa0/23 Root FWD 19 128.23 P2p
Fa0/24 Altn BLK 19 128.24 P2p

Now we will configure SW1 to become root for Vlan 10.

SW1(config)#spanning vlan 10 root primary

We can see that SW1 is now root for Vlan 10. Notice the priority is now 24576.

SW1(config)#do show spanning vlan 10

VLAN0010
Spanning tree enabled protocol ieee
Root ID Priority 24586
Address 0012.00cb.6c80
This bridge is the root
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Bridge ID Priority 24586 (priority 24576 sys-id-ext 10)
Address 0012.00cb.6c80
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300

Interface Role Sts Cost Prio.Nbr Type
—————- —- — ——— ——– ——————————–
Fa0/1 Desg FWD 19 128.1 P2p
Fa0/2 Desg FWD 19 128.2 P2p
Fa0/4 Desg FWD 19 128.4 P2p
Fa0/5 Desg FWD 19 128.5 P2p
Fa0/8 Desg FWD 19 128.8 P2p
Fa0/9 Desg FWD 19 128.9 P2p
Fa0/10 Desg FWD 19 128.10 P2p
Fa0/19 Desg FWD 19 128.19 P2p

Interface Role Sts Cost Prio.Nbr Type
—————- —- — ——— ——– ——————————–

Fa0/20 Desg FWD 19 128.20 P2p
Fa0/21 Desg FWD 19 128.21 P2p
Fa0/22 Desg FWD 19 128.22 P2p
Fa0/23 Desg FWD 19 128.23 P2p
Fa0/24 Desg FWD 19 128.24 P2p

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