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BeginnerPublished 2026-06-30

dot1Q Subinterfaces: The Router Side

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Practice creating 802.1Q subinterfaces on a single router uplink to deliver inter-VLAN routing using router-on-a-stick. The Layer-2 switch is already fully configured with VLAN 10 and VLAN 20, access ports for two hosts, and a working 802.1Q trunk to the router. Your job: leave the router's physical Ethernet0/0 unnumbered and add exactly two subinterfaces with the correct encapsulation tags and gateway IPs so hosts can reach their gateways and each other.

Track
Est. time
35 minutes
Difficulty
Beginner
Topics

Learning objectives

  • Configure router-on-a-stick subinterfaces that each carry their own VLAN tag on a single physical interface.
  • Keep the physical uplink unnumbered and enabled; place the gateway IPs on subinterfaces only.
  • Align encapsulation tags with VLAN IDs and understand how a mismatched or missing tag breaks reachability.
  • Verify inter-VLAN routing from hosts and with IOS show commands on both router and switch.

Troubleshooting focus

  • Identify and correct encapsulation tag mismatches (e.g., a subinterface's VLAN tag not equal to its VLAN ID).
  • Detect missing subinterfaces that leave a VLAN without a routed gateway.
  • Confirm that the physical parent interface is up, carries no IP address of its own, and is not otherwise misconfigured.
  • Differentiate switch-side issues from router-side configuration, using targeted show commands.

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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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