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How to configure Spanning Tree Protocol Root Primary and Root Secondary for Selecting Root Switch (Root Bridge)

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To configure a Switch to become the Spanning Tree Protocol (STP) Root Bridge (Root Switch) for a VLAN, you can use the “spanning-tree vlan <vlan-id> root” command from the Global Configuration Mode.

When you enter “spanning-tree vlan <vlan-id> root” command, IOS will check the Switch Priority values of the Root Bridges (Root Switches) for each VLAN. If all the switches in the VLAN have the same default priority, IOS will configure the Spanning Tree Priority value as 24576. If any Root Bridge (Root Switch) for the specified VLAN has a Switch Priority value lower than 24576, the switch sets its own priority for the specified VLAN to 4096 less than the lowest Spanning Tree Switch Priority value.

To configure a Switch as a Root Bridge (Root Switch) using "spanning-tree vlan <vlan_id> root primary" command, follow these steps.

omnisecu.com.SW1>enable
omnisecu.com.SW1#configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
omnisecu.com.SW1(config)#spanning-tree vlan 1 root primary
omnisecu.com.SW1(config)#exit
omnisecu.com.SW1#

To configure a Switch as a Secondary candidate for Root Bridge (Root Switch) using "spanning-tree vlan <van_id> root secondary" command follow these steps.

omnisecu.com.SW2>enable
omnisecu.com.SW2#configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
omnisecu.com.SW2(config)#spanning-tree vlan 1 root secondary
omnisecu.com.SW2(config)#exit
%SYS-5-CONFIG_I: Configured from console by console
omnisecu.com.SW2#exit

Cisco IOS Show command "show spanning-tree" will show the changed Switch Priority value as below.

omnisecu.com.SW1#show spanning-tree 
VLAN0001
  Spanning tree enabled protocol ieee
  Root ID    Priority    24577
             Address     000A.F379.A66C
             This bridge is the root
             Hello Time  2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    24577  (priority 24576 sys-id-ext 1)
             Address     000A.F379.A66C
             Hello Time  2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time  20

Interface        Role Sts Cost      Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Gi1/1            Desg FWD 4         128.25   P2p
Gi1/2            Desg FWD 4         128.26   P2p
              Jajish Thomason Google+
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