CCNA Break/Fix: The Branches the Core Cannot See

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Lab 1 of 13 in CCNA Break/Fix II: Read the Symptom · Next →

CCNA exam domain: IP Connectivity

Daily CCNA troubleshooting drill: Two branches can ping the core over their transit links, yet the core does not learn any branch routes. Your job is to trace how OSPF is enabled on each device, isolate why the core’s routing table is missing the branch LANs and loopbacks, and bring both branches into the correct OSPF area without changing how the core enables it.

Learning objectives

• Diagnose a reachability gap where connected pings work but dynamic routing does not populate the core
• Trace OSPF activation style per device and distinguish process-level vs interface-level enablement
• Isolate why an OSPF neighbor does not form despite working L3 transit
• Verify OSPF adjacencies and learned routes from the correct vantage points
• Compare the network statement method on the core with interface-mode enablement on the branches

Troubleshooting focus

• Start from the symptom: the host behind Branch A cannot reach core or remote loopbacks, but each branch can ping the core’s transit IP. That split points away from Layer 1/2 and toward IGP activation/scope.
• On the core, verify OSPF adjacencies and OSPF-learned routes. If adjacencies are down and no OSPF routes exist, check where OSPF is actually running on each side.
• On each branch, compare the intended design (transits and loopbacks in area 0) with what the device currently advertises. Focus on whether the interface participates in the process and area.
• Confirm you do not alter the core’s enablement method; apply fixes only at the branches according to the estate’s stated migration strategy.