STP Root Bridge and Blocked Path

Scenario

Three switches form a triangle. That redundancy would create a Layer-2 loop without STP. First observe the natural election, then make SW2 root primary and SW3 root secondary for VLAN 1.

Step 1 — Observe, do not change anything

SW1# show spanning-tree vlan 1
SW2# show spanning-tree vlan 1
SW3# show spanning-tree vlan 1

Write down the root bridge ID, each switch root port and any alternate/blocking port. The supplied notes explain the election using bridge priority first and MAC address as the tie-breaker.

Step 2 — Force the design

SW2# configure terminal
SW2(config)# spanning-tree vlan 1 root primary
SW2(config)# end

SW3# configure terminal
SW3(config)# spanning-tree vlan 1 root secondary
SW3(config)# end

Step 3 — Verify the new tree

SW2# show spanning-tree vlan 1
SW1# show spanning-tree vlan 1
SW3# show spanning-tree vlan 1

On the root bridge, active switch-to-switch ports should be designated. Non-root switches select a root port based on the best path toward the root. A redundant path is prevented from forwarding.

Step 4 — Fail a link

Shut one forwarding inter-switch link. Watch STP reconverge and confirm the previously redundant path can become usable. Restore the link and observe the tree again.

Break it

Set an unexpected lower STP priority on SW1. Without looking at the answer, predict the new root. Verify, explain why it won, then restore SW2 as primary.

Reference

Based on the supplied STP lab’s root-bridge, root-port, blocked-port and spanning-tree vlan 1 root primary exercises.