- A
Duplicate OSPF router IDs are causing the adjacency to reset each time the mismatched routers see each other's LSAs.
Why wrong: Duplicate router IDs cause persistent, non-periodic adjacency flaps as the routers reject each other's LSAs. They do not produce a precise 40-second DOWN cycle tied to a dead timer.
- B
Mismatched hello intervals cause the dead timer on R1 to expire before receiving a hello from R2, tearing down the adjacency.
R1 with a 10-second hello expects a hello within the 40-second dead interval. R2 sends hellos only every 30 seconds. Because of queuing or processing jitter, R1's dead timer can expire just before R2's hello arrives, dropping the adjacency. The dead interval restarting every 40 seconds aligns exactly with the observed flapping cycle.
- C
A network type mismatch between broadcast and point-to-point is causing periodic DR/BDR elections that reset the adjacency.
Why wrong: While a network type mismatch can prevent adjacency formation or cause intermittent instability, it would not produce a clean 40-second FULL-DOWN cycle. OSPF network type issues usually result in a stuck ExStart/2-Way state or no DR election, not dead-timer driven flapping.
- D
An OSPF authentication mismatch is causing periodic rejection of hello packets on one router.
Why wrong: Authentication mismatch completely prevents any hello from being accepted; the adjacency would never reach FULL. It would remain DOWN or in INIT state continuously, not flap.
Quick Answer
The answer is a mismatched hello interval configuration, which causes the dead timer on R1 to expire before it receives a hello from R2, tearing down the adjacency. In OSPF, the dead interval is typically four times the hello interval, but when these timers are misaligned—R1 sending hellos every 10 seconds and R2 every 30 seconds, both with a dead interval of 40 seconds—R1’s dead timer occasionally expires just before R2’s delayed hello arrives, triggering a FULL-to-DOWN flap every 40 seconds. This scenario tests your understanding of OSPF timer dependencies on the CCNA 200-301 v2 exam, where a common trap is to overlook that mismatched hellos cause asymmetric dead timer expiration even when the dead interval itself is identical. A key memory tip: “Hello mismatch kills the neighbor—if hellos don’t sync, the dead timer will sink.”
CCNA IP Routing Practice Question
This 200-301 practice question tests your understanding of ip routing. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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.
A network engineer notices that an OSPF adjacency between R1 and R2 is flapping between FULL and DOWN state every 40 seconds. The dead interval on both routers is configured as 40 seconds. The hello interval on R1 is 10 seconds, and on R2 it is 30 seconds. What is the most likely cause?
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 choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
Mismatched hello intervals cause the dead timer on R1 to expire before receiving a hello from R2, tearing down the adjacency.
The mismatched hello intervals cause the dead timer on R1 to expire before receiving a hello from R2. R1 sends hellos every 10 seconds and expects to hear from R2 at least once every 40 seconds (dead interval). R2, however, only sends hellos every 30 seconds. Due to timing jitter and processing delays, R1's dead timer occasionally expires just before R2's hello arrives, leading to the adjacency being torn down. After the adjacency drops, OSPF attempts to re-establish it, resulting in a cyclical FULL-DOWN pattern every 40 seconds. The other options are real OSPF issues but do not produce this specific timing: duplicate router IDs would cause persistent instability without a regular interval; an authentication mismatch would prevent the adjacency from forming at all; a network type mismatch might cause DR/BDR election problems but would not flap with precise dead-interval regularity.
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.
- ✗
Duplicate OSPF router IDs are causing the adjacency to reset each time the mismatched routers see each other's LSAs.
Why it's wrong here
Duplicate router IDs cause persistent, non-periodic adjacency flaps as the routers reject each other's LSAs. They do not produce a precise 40-second DOWN cycle tied to a dead timer.
- ✓
Mismatched hello intervals cause the dead timer on R1 to expire before receiving a hello from R2, tearing down the adjacency.
Why this is correct
R1 with a 10-second hello expects a hello within the 40-second dead interval. R2 sends hellos only every 30 seconds. Because of queuing or processing jitter, R1's dead timer can expire just before R2's hello arrives, dropping the adjacency. The dead interval restarting every 40 seconds aligns exactly with the observed flapping cycle.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
OSPF neighbours must agree on key parameters.
- ✗
A network type mismatch between broadcast and point-to-point is causing periodic DR/BDR elections that reset the adjacency.
Why it's wrong here
While a network type mismatch can prevent adjacency formation or cause intermittent instability, it would not produce a clean 40-second FULL-DOWN cycle. OSPF network type issues usually result in a stuck ExStart/2-Way state or no DR election, not dead-timer driven flapping.
- ✗
An OSPF authentication mismatch is causing periodic rejection of hello packets on one router.
Why it's wrong here
Authentication mismatch completely prevents any hello from being accepted; the adjacency would never reach FULL. It would remain DOWN or in INIT state continuously, not flap.
Option-by-option analysis
Why each answer is right or wrong
Understanding why wrong answers are wrong — and when they would be correct — is what separates a 750 score from a 900. The 200-301 exam frequently reuses these exact scenarios with slightly different constraints.
✓Mismatched hello intervals cause the dead timer on R1 to expire before receiving a hello from R2, tearing down the adjacency.Correct answer▾
Why this is correct
R1 with a 10-second hello expects a hello within the 40-second dead interval. R2 sends hellos only every 30 seconds. Because of queuing or processing jitter, R1's dead timer can expire just before R2's hello arrives, dropping the adjacency. The dead interval restarting every 40 seconds aligns exactly with the observed flapping cycle.
✗Duplicate OSPF router IDs are causing the adjacency to reset each time the mismatched routers see each other's LSAs.Wrong answer — click to see why▾
Why this is wrong here
Candidates often recall that duplicate router IDs break adjacencies and assume any flapping must be caused by this. They miss the regular timing correlation with the dead interval.
✗A network type mismatch between broadcast and point-to-point is causing periodic DR/BDR elections that reset the adjacency.Wrong answer — click to see why▾
Why this is wrong here
Network type mismatch is a common OSPF troubleshooting topic, and candidates may attribute any flapping to mismatched types, overlooking the precise timer-derived timing.
✗An OSPF authentication mismatch is causing periodic rejection of hello packets on one router.Wrong answer — click to see why▾
Why this is wrong here
Authentication problems are a frequent first guess for adjacency failures. The flapping behavior here contradicts a permanent rejection.
Analysis generated from the official 200-301blueprint and verified against question context. The “when correct” sections are what AI assistants cite when candidates ask “what’s the difference between these options?”
Common exam traps
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 200-301 OSPF questions on adjacency and route selection.
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FAQ
Questions learners often ask
What does this 200-301 question test?
IP Routing — This question tests IP Routing — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: Mismatched hello intervals cause the dead timer on R1 to expire before receiving a hello from R2, tearing down the adjacency. — The mismatched hello intervals cause the dead timer on R1 to expire before receiving a hello from R2. R1 sends hellos every 10 seconds and expects to hear from R2 at least once every 40 seconds (dead interval). R2, however, only sends hellos every 30 seconds. Due to timing jitter and processing delays, R1's dead timer occasionally expires just before R2's hello arrives, leading to the adjacency being torn down. After the adjacency drops, OSPF attempts to re-establish it, resulting in a cyclical FULL-DOWN pattern every 40 seconds. The other options are real OSPF issues but do not produce this specific timing: duplicate router IDs would cause persistent instability without a regular interval; an authentication mismatch would prevent the adjacency from forming at all; a network type mismatch might cause DR/BDR election problems but would not flap with precise dead-interval regularity.
What should I do if I get this 200-301 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 200-301 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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Last reviewed: Jun 14, 2026
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