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IntermediatePublished 2026-06-30

Native VLAN Mismatch: Diagnosing a Broken Trunk with CDP

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Diagnose and remediate a trunk misconfiguration between an access switch and a distribution switch so that same-VLAN hosts across two access switches can communicate end-to-end. Use CDP and trunk verification commands to investigate the fault and restore proper trunk operation, without introducing any Layer-3 routing.

Track
Est. time
45 minutes
Difficulty
Intermediate
Topics

Learning objectives

  • Identify a native VLAN mismatch using CDP neighbor details
  • Explain untagged/native VLAN behavior on 802.1Q trunks
  • Align native VLANs consistently across trunk endpoints
  • Verify trunk state, native VLAN, and allowed VLAN lists
  • Validate same-VLAN host connectivity across multiple access switches

Troubleshooting focus

  • Use show cdp neighbors detail to detect native VLAN mismatches
  • Correlate show interfaces trunk and show interfaces switchport to confirm native and allowed VLANs
  • Test end-to-end reachability from the hosts before and after the fix

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

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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