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IntermediatePublished 2026-06-30

CCNA VLAN Trunking 7: Trunking Across Three Switches

Bundle lab

Build an end-to-end 802.1Q path across three Cisco IOS Layer-2 switches so VLAN 10 transports user traffic from an access port on the left switch to an access port on the right switch through a middle switch. Harden trunks (native VLAN 999, nonegotiate) and verify with show interfaces trunk. Then intentionally break the allow-list to see the outage and restore service.

Track
Est. time
45 minutes
Difficulty
Intermediate
Topics

Learning objectives

  • Configure access ports and create VLANs on Layer-2 switches
  • Build two 802.1Q trunks that carry VLAN 10 end-to-end across three switches
  • Harden trunks by setting a non-default native VLAN and disabling DTP negotiation
  • Verify trunk and VLAN transport state with show interfaces trunk
  • Diagnose and fix an outage caused by an incomplete trunk allowed-VLAN list

Troubleshooting focus

  • Identify where VLAN transport breaks across a multi-switch path using show interfaces trunk and show vlan brief
  • Detect and correct a native VLAN mismatch across trunks
  • Spot a missing VLAN in a trunk allowed list and restore it
  • Verify correct host access VLAN assignment and portfast/bpduguard on access ports

Topology

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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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