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

Command

Spanning-Tree VLAN Root Secondary

Use

This command sets the switch to become backup root for a given VLAN. It works by lowering the priority of the switch until it becomes one priority higher then the root, so it will take over if the root fails. Once the switch is root it will not prevent any other switch from becoming root.

Syntax

Switch(config)#ispanning-tree vlan <vlan range> root secondary

Example

In the below example we will configure SW2 to become the backup root for VLAN 10. Currently, we see that SW2 is not root for VLAN 10. Notice the priority is currently 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

Now we will configure SW2 to become the backup root for VLAN 10.

SW2(config)#spanning vlan 10 root secondary

We can see that SW2 is now has a priority of 28672.

SW2(config)#do show spanning vlan 10

VLAN0010
Spanning tree enabled protocol ieee
Root ID Priority 24586
Address 0012.00cb.6c80
Cost 19
Port 25 (FastEthernet0/23)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Bridge ID Priority 28682 (priority 28672 sys-id-ext 10)
Address 0019.0606.b600
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 19 128.26 P2p

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