IntermediatePublished 2026-09-18
CCNA Break/Fix: Three Circuits That Test Clean and Carry Nothing
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Lab 4 of 13 in CCNA Break/Fix II: Read the Symptom · ← Previous · Next →
CCNA exam domain: Network Access
A replacement branch router came up on three carrier handoffs. All subinterfaces and /30s look correct and line protocols are up, yet no traffic passes. Your job: read what each circuit actually puts on the wire and bring the branch into alignment with the remote ends — including the carrier-published IP MTU on one circuit — without altering addressing or static routes.
Learning objectives
- Diagnose a mismatch between subinterface 802.1Q tags and the service order
- Trace traffic flow from the end host through a router-on-a-stick WAN handoff
- Isolate path failures caused by MTU/fragmentation versus addressing and routes
- Compare working and non-working circuits using show commands that reveal encapsulation and MTU
- Verify end-to-end reachability from the user LAN after repairs
Troubleshooting focus
- Validate end-to-end from the host first; a router-to-router ping may work even if host return routes or MTU are wrong.
- On each WAN subinterface, compare the 802.1Q tag used on the wire with what the service order specifies.
- Check that each carrier circuit using a reduced IP MTU has consistent values on both ends.
- Confirm static routes point to the correct next hops and that reverse routes exist for the host LAN.
- Use ping and traceroute with and without larger payloads to distinguish MTU problems from routing issues.
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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.
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