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