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AdvancedPublished 2026-06-30

Trunk Verification & Diagnosing a Silent VLAN

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An advanced CCNA switching lab focused on verifying 802.1Q trunks and diagnosing a silent VLAN after a switch refresh. The topology uses three layer-2 switches (a distribution switch between two access closets) and two Alpine hosts, and users in VLAN 20 at one closet cannot reach their VLAN 20 peers at the other closet across the trunk path. Learners use show interfaces trunk, show interfaces switchport, and show vlan brief to locate the break and restore predictable Layer-2 forwarding without adding any Layer-3 configuration.

Track
Est. time
55 minutes
Difficulty
Advanced
Topics

Learning objectives

  • Configure and verify 802.1Q trunks on layer-2 switches
  • Interpret show interfaces trunk, show interfaces switchport, and show vlan brief to locate trunk/VLAN faults
  • Align native VLANs and trunk allowed VLAN lists across a multi-switch path
  • Validate end-host access VLAN placement and expected same-VLAN isolation/forwarding
  • Apply trunk hardening practices (pin the native VLAN to 999, disable trunk renegotiation, keep VLAN 1 off the trunk)

Troubleshooting focus

  • Identify a silent VLAN caused by an incomplete trunk allowed VLAN list
  • Recognize and remediate a native VLAN mismatch across trunk ends
  • Detect a host interface placed in the wrong access VLAN

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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