CCNA Break/Fix: Both Branches Went Quiet at 02:14

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Lab 6 of 13 in CCNA Break/Fix II: Read the Symptom · ← Previous · Next →

CCNA exam domain: IP Connectivity

An overnight hardening change reached the core but not the branches. By morning, both OSPF adjacencies to the core were down even though every point-to-point link still pings and no addressing changed. Restore end-to-end reachability by diagnosing what the core and each spoke now disagree about on the carrier-facing transits and finishing the change on the branch ends.

Learning objectives

• Diagnose control-plane loss when data-plane pings still succeed on transit links
• Trace OSPF adjacency formation and isolate neighbor-mismatch causes at the interface level
• Distinguish area, timers, MTU, and security parameters that must match on both OSPF neighbors
• Compare the intended design for carrier-facing transits against current interface settings
• Verify restored adjacencies and end-to-end reachability from the user host

Troubleshooting focus

• Use show ip ospf neighbor and show ip ospf interface on each side of the transit to confirm whether the adjacency is forming and to compare per-interface parameters (area, network type, timers, MTU, and security).
• Confirm the data plane is intact with direct pings across each point-to-point /30; if those work while OSPF remains DOWN, focus on OSPF neighbor requirements rather than L1/L2.
• Check that LAN-facing interfaces are passive under OSPF to avoid false leads from missing neighbors on user segments.
• After repair, verify that the branch LAN route is present in the core LSDB and that the host can reach remote loopbacks.