The correct answer is that the LSA refresh interval has expired. OSPF LSAs are periodically refreshed every 1800 seconds, or 30 minutes, by default to ensure link-state database synchronization and to prevent stale routes from lingering, even when no topology changes occur. This behavior is a fundamental OSPF mechanism, not a network issue or misconfiguration. On the Cisco SPCOR / CCNP Service Provider Core 350-501 exam, this concept tests your understanding of OSPF’s built-in reliability features; a common trap is assuming that periodic flooding indicates a problem, when in fact it is expected and necessary. Remember that the LSA refresh interval is half of the MaxAge timer (3600 seconds), so a simple memory tip is “refresh at 30, flush at 60”—the 30-minute refresh keeps LSAs alive before they would otherwise age out.
350-501 Networking Practice Question
This 350-501 practice question tests your understanding of networking. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
An OSPF network uses point-to-point links. The engineer notices that LSAs are being flooded every 30 minutes even when no topology changes occur. What is the most likely reason?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The LSA refresh interval has expired.
Option B is correct because OSPF LSAs are periodically refreshed every 1800 seconds (30 minutes) by default. Option A is incorrect; this flooding is expected. Option C is incorrect; LSUs are used for flooding LSAs. Option D is incorrect; the LS refresh interval is 30 minutes, not a timer issue.
Key principle: OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
The routers are using LSU packets incorrectly.
Why it's wrong here
Incorrect. LSUs are normal for flooding LSAs.
✗
There is a flapping interface on the network.
Why it's wrong here
Incorrect. Flapping would cause frequent updates, not a regular 30-minute pattern.
✗
The dead timer is set too low.
Why it's wrong here
Incorrect. Dead timer affects neighbor state, not LSA flooding.
Common exam trap: OSPF can fail even when IP connectivity looks correct
OSPF neighbour formation depends on matching areas, timers, network type, authentication and passive-interface behaviour. Do not choose an answer only because the devices can ping.
Detailed technical explanation
How to think about this question
OSPF questions usually test the details that control adjacency and route selection. Read the neighbour state, area, router ID and interface configuration before deciding what is wrong.
KKey Concepts to Remember
OSPF neighbours must agree on key parameters.
Router ID selection can affect neighbour relationships and LSDB output.
OSPF cost influences the preferred path.
A route can appear in OSPF information but not become the installed route.
TExam Day Tips
→Check area mismatch first when OSPF adjacency fails.
→Review passive interfaces when a network is advertised but no neighbour forms.
→Use show ip ospf neighbor and show ip route clues carefully.
Key takeaway
OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.
Real-world example
How this comes up in practice
A network engineer at a university connects two campus buildings via a fibre link. Both routers run OSPF, but no adjacency forms — even though both routers can ping each other. The engineer finds one router is in area 0 and the other in area 1. OSPF adjacency requires matching area numbers, hello/dead timers, and network type. IP reachability alone is not enough.
What to study next
Got this wrong? Here's your next step.
Review OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related 350-501 OSPF questions on adjacency and route selection.
Networking — This question tests Networking — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: The LSA refresh interval has expired. — Option B is correct because OSPF LSAs are periodically refreshed every 1800 seconds (30 minutes) by default. Option A is incorrect; this flooding is expected. Option C is incorrect; LSUs are used for flooding LSAs. Option D is incorrect; the LS refresh interval is 30 minutes, not a timer issue.
What should I do if I get this 350-501 question wrong?
Review OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related 350-501 OSPF questions on adjacency and route selection.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
OSPF neighbours must agree on key parameters.
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