Spanning Tree Protocol Root Switch (Root Bridge)

 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" command, follow these steps.

1. Enter into configuration mode on SW1

2. Issue the command "spanning-tree vlan <vlan-id> root" that modifies the switch priority from the default 32768 to a lower value to allow the switch to become the root switch for VLAN 1 

3. Verify the configuration using “show spanning-tree” command.

SW1>enable 
SW1#configure terminal 
SW1(config)#spanning-tree vlan 1 root 
SW1(config)#exit
SW1#show spanning-tree

Please visit here for labs on Spanning tree protocols and other switch labs on Junos and IOS : Available Switch Simulator labs  

 

Also check this for  Switch Network Simulator for JUNOS and IOS