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AdvancedPublished 2026-06-30

802.1Q Trunking Troubleshooting Capstone

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Advanced CCNA switching capstone centered on restoring end-to-end VLAN 20 transport across three Layer-2 switches using 802.1Q trunks. The starter ships intentionally broken: after a simulated maintenance window, two Alpine hosts in VLAN 20 can no longer reach each other across the inter-switch trunks. Learners diagnose with show interfaces trunk, show interfaces switchport, and show vlan brief, then identify and correct the trunking faults in the right order and verify with host pings.

Track
Est. time
55 minutes
Difficulty
Advanced
Topics

Learning objectives

  • Trace end-to-end VLAN 20 transport across multiple L2 switches and verify trunk state.
  • Identify and remediate a DTP mode combination that prevents trunk formation (auto/auto).
  • Detect and correct a native VLAN mismatch on a trunk.
  • Audit and fix a trunk allowed VLAN list that omits a required VLAN.
  • Differentiate host access VLAN correctness from trunk transport failures.

Troubleshooting focus

  • An inter-switch port is missing from show interfaces trunk: inspect each end with show interfaces switchport. Two ends both reading "Administrative Mode: dynamic auto" never form a trunk — hard-set trunk mode.
  • HOST-A still cannot ping 10.20.20.20 after the trunks form: compare the native VLAN at both ends in show interfaces trunk. A mismatch misclassifies untagged frames.
  • VLAN 20 is absent from "Vlans allowed and active": fix the allow-list at both ends. One-sided pruning alone breaks end-to-end reachability.
  • Trunks look right but the hosts stay isolated: check the host-facing ports in show vlan brief. Both must be access ports in VLAN 20.
  • Pings succeed but the lab scores short: confirm you added no SVI or routed interface. This segment is L2-only by design.

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Learn VLANs, Trunking & Inter-VLAN RoutingWhat Is a VLAN? VLANs Explained for Beginners

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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