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BeginnerPublished 2026-08-24

CCNA Break/Fix: The Trunk That Carries Only Some VLANs

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A compact two-switch, two-host CML lab focused on troubleshooting an 802.1Q trunk. One VLAN traverses the trunk, another does not. Learners must read trunk state on both ends, compare against the intended design, and correct the mismatch without disrupting the working VLAN. Includes a second subtle trunk hygiene fault (native VLAN mismatch) that must be standardized.

Track
Est. time
35 minutes
Difficulty
Beginner
Topics

Learning objectives

  • Diagnose asymmetric trunk behaviour where one VLAN forwards and another does not
  • Use show interfaces trunk, VLAN tables, and per-port status to isolate per-end trunk discrepancies
  • Correct the allowed VLAN list without disturbing existing working traffic
  • Standardize the native VLAN on an 802.1Q trunk to a campus parking VLAN

Troubleshooting focus

  • When one VLAN forwards and another does not across a trunk, inspect the trunk state on both ends and compare native VLAN and allowed VLANs to the intended design
  • Validate that access ports on both switches are in the correct VLANs and are up/up before changing trunk parameters
  • Confirm that VLANs exist locally on each L2 switch and appear active, then correlate with the trunk's 'Allowed' and 'Active' columns

Topology

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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.

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