AdvancedPublished 2026-09-24
CCNA Break/Fix: Subnets That Went Dark After Summary Tidy
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Lab 10 of 13 in CCNA Break/Fix II: Read the Symptom · ← Previous · Next →
CCNA exam domain: IP Connectivity
Two internal subnets behind a branch router stopped working after a summarization tidy-up. Everything else inside the same /16 summary is fine, and the edge still has a discard route for the summary. Your job: trace which route each hop actually matches for those destinations, then restore reachability without removing or weakening the summary.
Learning objectives
- Diagnose reachability that fails only for specific subnets while others in the same summary work
- Trace per-hop longest-prefix-match decisions for specific destinations
- Isolate where traffic is leaving the intended path and being dropped
- Distinguish summary routes from required more-specifics in a hub-and-spoke static design
- Compare the routing table vs next-hop recursion to validate path selection
- Verify end-to-end repair from the client hosts, including router RIB checks
Troubleshooting focus
- Only two subnets behind one router are unreachable; others inside the same 172.16.0.0/16 summary still work
- A summary route exists toward the WAN, and the edge keeps a discard for the same summary
- Packets for the failed subnets follow the summary at the core and are dropped near the edge
- Success requires ensuring a more-specific path exists everywhere that could otherwise match the broader summary
- Validate repairs with longest-prefix-match tools (show ip route <IP>, show ip cef <IP>) at each hop
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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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