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Half of the company's sites lost connectivity to a specific subnet within seconds of each other. The core routers show the route, but traffic to the subnet times out. Pings from the core work. Pings from the branch routers fail. Something changed at the core that made its advertisement invalid downstream. The root cause: an OSPF area mismatch was pushed to one ABR during a routine change — the ABR stopped advertising routes into that area.
Routing problems present in two ways: missing routes (no path) or wrong routes (path to nowhere). Both are visible with `show ip route`, but the specific diagnosis depends on which protocol is failing and how. This lesson covers the common cases.
Users at a branch report intermittent connectivity to a subnet at HQ. Ping sometimes works, sometimes fails. `show ip route` on the branch router shows the route via OSPF. `show ip ospf neighbor` shows the neighbor as FULL. But when you check the HQ router that owns the subnet, its OSPF neighbor state to the branch's area ABR just went from FULL to EXSTART twice in the last hour.
Select the safest next action.
Routing loops happen when two routers point at each other for the same destination. Traffic bounces between them until TTL expires. Symptoms: high interface counters between two routers, traceroute showing the same two IPs repeatedly, and high CPU on both routers. Diagnosis: `show ip route <prefix>` on both ends — if A says 'next-hop is B' and B says 'next-hop is A,' you have a loop. Usually caused by a misconfigured static route or a bad redistribute.
The most common OSPF troubleshooting mistake: jumping to debug before checking the basics. 90% of OSPF problems are visible with three commands — `show ip ospf neighbor`, `show ip ospf interface brief`, and `show ip route ospf`. Compare those across routers before enabling debug. The second most common mistake is not comparing both ends of a link — one-sided config checks miss half the problem.
Finish the lesson once you have worked through the material. This awards ★ 40 XP.
Select each vulnerable line, then classify the finding.
Findings
Review target: 16 lines · 1 expected finding