Root Bridge Configuration Lab

You are the network administrator, and three switches sit in a triangle with redundant links between them.
Without Spanning Tree, that redundancy turns into a Layer 2 loop that floods every port and slows the network to a crawl.

Figure 1 – A Layer 2 loop before anyone controls the election

STP already runs by default, but it elected the Root Bridge on its own, with no plan behind it.
In this lab, you take control of that election and decide which switch is Root, which is the backup, and which stays non-root.

The Target Design

Here is the target design for this lab: SW1 Root Bridge, SW2 Backup Root Bridge, SW3 non-root.

Figure 2 – Target design: Root, Backup, and non-root switches

With nothing configured, PVST+ is already running on every switch and the election already happened by default, purely on MAC address.

Figure 3 – The three-switch lab before any configuration

Confirming the Default Root

You did not choose a Root, yet one already exists.
Check SW2 first to see who holds the role right now:

SW2# show spanning-tree vlan 1
VLAN0001
  Spanning tree enabled protocol ieee
  Root ID    Priority    32769
             Address     0001.63A6.E107
             This bridge is the root
             Hello Time  2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32769  (priority 32768 sys-id-ext 1)
             Address     0001.63A6.E107
             Hello Time  2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time  20

Interface        Role Sts Cost      Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Gi0/1            Desg FWD 4         128.25   P2p
Gi0/2            Desg FWD 4         128.26   P2p

SW2 ended up Root purely by chance in your lab, because its MAC address happened to be the lowest.
That is exactly the problem: nothing here was engineered.

Figure 4 – Initial topology with SW2 as the unplanned Root

Where SW3 Blocks by Default

SW3 lost the election, so it had to block one port on its own to keep the topology loop-free.
Check which one:

SW3# show spanning-tree vlan 1
VLAN0001
  Spanning tree enabled protocol ieee
  Root ID    Priority    32769
             Address     0001.63A6.E107
             Cost        4
             Port        26(GigabitEthernet0/2)
             Hello Time  2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32769  (priority 32768 sys-id-ext 1)
             Address     0090.2BE2.8EE9
             Hello Time  2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time  20

Interface        Role Sts Cost      Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Gi0/1            Altn BLK 4         128.25   P2p
Gi0/2            Root FWD 4         128.26   P2p

SW3 sees SW2 as Root and blocked Gi0/1, its link toward SW1.
Nothing planned that: the blocked port landed wherever the default election left it.

Keep that port in mind, it will move once you make SW1 the Root.

Here is how to take control of the election.

Priority Values

Cisco lets you set the priority manually, from 0 to 61440 in increments of 4096.
The lower the value, the more likely the switch becomes Root.

Figure 5 – Bridge ID recap: priority and MAC decide the winner