Goldfish Networks wordmark
AdvancedPublished 2026-07-02

STP 10: Spanning Tree Troubleshooting Capstone

Bundle lab

Advanced Rapid-PVST+ troubleshooting on a 3-switch triangle with a real Layer-2 loop. Two deliberate faults are seeded: an unintended root bridge wins VLAN 100 due to a mis-set/default priority, and PortFast/BPDU Guard are mistakenly applied on an inter-switch trunk. Two Alpine hosts in VLAN 100 verify user impact. Your job: use show commands to diagnose, then restore the correct root and remove edge features from the trunk while preserving them on access ports.

Track
Est. time
70 minutes
Difficulty
Advanced
Topics

Learning objectives

  • Identify the operational root bridge and spanning-tree port roles for VLAN 100 using show spanning-tree outputs.
  • Restore deterministic root control by setting an explicit bridge priority so SW1 is the root for VLAN 100.
  • Detect and remove PortFast/BPDU Guard from an inter-switch trunk while keeping them on access ports only.
  • Confirm Rapid-PVST+ mode is active and that a single port blocks to break the physical loop.
  • Validate end-to-end host reachability in VLAN 100 after remediation and interpret why the path changed.

Troubleshooting focus

  • If hosts in the same VLAN have intermittent or asymmetric reachability, verify who the root bridge is (priority and MAC) and compare it to design intent.
  • On every inter-switch link, confirm the interface is a trunk and not treated as an edge: check edge/PortFast flags and any guards that could err-disable the port on BPDU receipt.
  • Look for a link reduction (an intended triangle showing as a V) caused by an err-disabled trunk; confirm the reason and recover cleanly.
  • Ensure access ports are the only place PortFast/BPDU Guard is applied; verify the correct VLAN is assigned to each access port.
  • Review the spanning-tree summary and per-VLAN detail outputs for consistent Rapid-PVST+ operation, role/state convergence, and a single blocking port in the triangle.

Topology

Subscribe to preview this lab's topology.

See plans

Related labs

More hands-on Cisco Modeling Labs practice on the same track and topics.

Study this topic

Free guides, a command cheat sheet, and graded practice for this lab's topic.

Learn Spanning Tree (STP)What Is Spanning Tree Protocol (STP)?

Grade your work

How this lab is graded
  • Build it your way. Where a lab lets you choose a value — a VLAN name, an interface description — grading checks that you configured it, not which name you picked. Names that another line has to reference, like an ACL applied with access-class, are stated in the guide and do have to match.
  • Addresses, modes and protocol keywords are exact. An IP address, a subnet mask, switchport mode trunk, an encapsulation — these carry the meaning of the lab, so they are graded as written in the guide.
  • Grading reads your saved configuration. Export the lab from CML after you have configured it, and make sure anything you set is in the running-config — a change that only exists in a terminal session never reaches the grader.
  • You can submit as many times as you like. Your best score stands, and each attempt tells you which checks passed so you can work the gaps.
  • Scored something you believe is correct? Use Report an issue on this page — that is exactly how the grading fixes in the changelog got found.

Create a free account to submit your lab for grading.

Create a free account
Found a problem with this lab?

Please sign in to report a problem — tying it to your attempts lets us reproduce and fix it faster.