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AdvancedPublished 2026-06-25

VLAN Troubleshooting Capstone: Native Mismatch & VLAN Drift

Bundle lab

Advanced CCNA L2 troubleshooting in a compact branch. Some PCs can't reach their default gateway or other departments across two switches and a router-on-a-stick gateway, and your monitoring system has flagged a switching/trunking problem on the path. Validation is performed from the endpoints (Alpine hosts) using pings and ARP, and trunks are hardened with a non-default native VLAN.

Track
Est. time
65 minutes
Difficulty
Advanced
Topics

Learning objectives

  • Identify and correct access-port VLAN mismatches that break host connectivity
  • Detect and remediate native VLAN mismatches on 802.1Q trunks
  • Fix incomplete trunk allowed-VLAN lists to restore VLAN reachability
  • Differentiate access vs trunk roles and convert links accordingly
  • Validate segmentation using host-based tests (ping and ARP) and switch show commands
  • Harden trunks with an intentional non-default native VLAN

Troubleshooting focus

  • User access port assigned to the wrong VLAN (VLAN drift)
  • Switch-to-switch trunk formed but native VLAN mismatch (CDP warnings)
  • Trunk allowed VLAN list pruned incorrectly (required VLAN 20 missing)
  • Uplink toward router mis-set as access, preventing ROAS subinterfaces from forwarding

Topology

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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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