Hands-On Lab: Verifying Spanning Tree Port States on Catalyst Switches

Lab Objective

Observe a switch interface progress through Spanning Tree Protocol's port states after being enabled, and use show commands to identify a port's current state and role at any given moment.

Lab Purpose

Understanding STP port states is essential both for recognizing normal, expected link startup delay and for diagnosing situations where a port remains stuck in an unexpected state, such as permanently blocking when it should be forwarding.

Lab Topology

Switch1 ---- Gi1/0/5 ---- Switch2

A redundant second link also exists between
Switch1 and Switch2 via Gi1/0/6, creating a
loop that STP must resolve

Task 1: Verify Current Spanning Tree Operation

Use a show command to display the current Spanning Tree state and role of both Gi1/0/5 and Gi1/0/6 on Switch1.

Task 2: Trigger a State Transition

Shut down and then re-enable Gi1/0/5, and immediately begin checking its state repeatedly to observe the transition sequence.

Task 3: Identify the Port Roles

Determine which of the two redundant links is in the forwarding state (either root or designated) and which is blocking.

Task 4: Confirm the Final Stable State

After allowing sufficient time for convergence, confirm both ports have reached their final stable state.

Solution and Verification

Switch1# show spanning-tree vlan 1

Interface     Role  Sts  Cost   Prio.Nbr  Type
Gi1/0/5       Desg  FWD  4      128.5     P2p
Gi1/0/6       Altn  BLK  4      128.6     P2p

Switch1(config)# interface gigabitethernet1/0/5
Switch1(config-if)# shutdown
Switch1(config-if)# no shutdown

Switch1# show spanning-tree interface gigabitethernet1/0/5

-- Checked repeatedly over roughly 30-50 seconds:

Interface        Role Sts Cost
Gi1/0/5          Desg BLK 4      (immediately after no shutdown)
Gi1/0/5          Desg LIS 4      (a few seconds later)
Gi1/0/5          Desg LRN 4      (a few seconds after that)
Gi1/0/5          Desg FWD 4      (finally stable, ~30-50 sec total)

Switch1# show spanning-tree vlan 1

Interface     Role  Sts  Cost   Prio.Nbr  Type
Gi1/0/5       Desg  FWD  4      128.5     P2p
Gi1/0/6       Altn  BLK  4      128.6     P2p
-- Gi1/0/5 is Designated/Forwarding (actively
-- carrying traffic), while Gi1/0/6 remains
-- Alternate/Blocking (physically connected but
-- not forwarding, ready as backup) -- this is
-- exactly the expected result of STP resolving
-- the redundant loop between the two switches

Key Takeaway

A port cycling through Blocking, Listening, Learning, and finally Forwarding after being enabled is completely normal behavior taking up to 30-50 seconds with classic 802.1D STP — a port that never progresses beyond Blocking or Listening after this delay window signals a genuine problem worth investigating further, rather than simply needing more time.