Goldfish Networks wordmark
BeginnerPublished 2026-08-25

CCNA Break/Fix: The Root Bridge Nobody Elected

Archive lab

Daily CCNA troubleshooting lab focused on Spanning Tree root bridge selection. A fully working campus access/distribution triangle forwards traffic, but the wrong switch is root for VLAN 10 and VLAN 20, forcing an inefficient path. Diagnose with show commands and repair by influencing the election without creating a loop.

Track
Est. time
45 minutes
Difficulty
Beginner
Topics

Learning objectives

  • Use show spanning-tree to identify the current root and port roles per VLAN
  • Correlate trunks, allowed VLANs, and native VLAN configuration with STP topology
  • Influence STP per-VLAN root selection by setting bridge priority
  • Restore the intended design safely without introducing loops
  • Verify convergence and that traffic takes the intended shortest path

Troubleshooting focus

  • Identify symptoms of an unintended STP root (unexpected root ID, alternate/blocked links on a direct path)
  • Distinguish STP behavior per VLAN and confirm both VLAN 10 and VLAN 20 trees
  • Validate trunk consistency (mode, allowed list, native VLAN) before changing STP priorities
  • Plan a safe priority change that clearly beats the current root without flapping ports

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.

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.