- A
The ABR is filtering the specific /24 routes using a distribute-list under the OSPF process.
Why wrong: No distribute-list was configured in the scenario. The symptom aligns with the default behavior of the area range command, which suppresses specific routes without any additional filtering.
- B
The routers in Area 1 are no longer advertising their /24 routes to the ABR because the ABR is in a different area.
Why wrong: OSPF routers in Area 1 continue to flood their Type-1 LSAs to the ABR, which then converts them to Type-3 LSAs. The ABR receives all /24 routes before summarization occurs, so this is not the cause.
- C
The area range command causes the ABR to advertise only the summary LSA and suppress the more-specific Type-3 LSAs for the range.
By default, the area range command summarizes the specified prefix range and suppresses the individual component routes from being advertised into the target area. Only the summary LSA appears in Area 0, which matches the observed behavior.
- D
The ABR has automatically created a discard route to null0 for the summary, which is dropping all traffic destined to the summarized networks.
Why wrong: Although a discard route for 10.0.0.0/16 is installed on the ABR to prevent routing loops, it only discards traffic when no more-specific route matches. Since the ABR still has the /24 routes from Area 1, traffic to 10.0.5.100 is forwarded using the longer match and is not dropped.
CCNA IP Routing Practice Question
This 200-301 practice question tests your understanding of ip routing. 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.
After configuring the area 0 range 10.0.0.0 255.255.0.0 command on an OSPF ABR, a technician finds that a host at 10.0.5.100 in Area 1 cannot reach hosts in Area 0. The ABR’s OSPF database shows only the summary 10.0.0.0/16 in Area 0, and no individual /24 routes. 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
The area range command causes the ABR to advertise only the summary LSA and suppress the more-specific Type-3 LSAs for the range.
The area 0 range command on an OSPF ABR creates a summary Type-3 LSA and suppresses the more-specific Type-3 LSAs within that range. This is expected OSPF route summarization behavior; only the summary route is advertised into Area 0, which explains why the /24 routes are missing. The ABR continues to forward traffic properly because it retains specific routes learned from Area 1, so connectivity within the summary should still work under normal conditions. The other answers either describe non-existent configurations or misunderstand the discard route's role.
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 ABR is filtering the specific /24 routes using a distribute-list under the OSPF process.
Why it's wrong here
No distribute-list was configured in the scenario. The symptom aligns with the default behavior of the area range command, which suppresses specific routes without any additional filtering.
- ✗
The routers in Area 1 are no longer advertising their /24 routes to the ABR because the ABR is in a different area.
Why it's wrong here
OSPF routers in Area 1 continue to flood their Type-1 LSAs to the ABR, which then converts them to Type-3 LSAs. The ABR receives all /24 routes before summarization occurs, so this is not the cause.
- ✓
The area range command causes the ABR to advertise only the summary LSA and suppress the more-specific Type-3 LSAs for the range.
Why this is correct
By default, the area range command summarizes the specified prefix range and suppresses the individual component routes from being advertised into the target area. Only the summary LSA appears in Area 0, which matches the observed behavior.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
OSPF neighbours must agree on key parameters.
- ✗
The ABR has automatically created a discard route to null0 for the summary, which is dropping all traffic destined to the summarized networks.
Why it's wrong here
Although a discard route for 10.0.0.0/16 is installed on the ABR to prevent routing loops, it only discards traffic when no more-specific route matches. Since the ABR still has the /24 routes from Area 1, traffic to 10.0.5.100 is forwarded using the longer match and is not dropped.
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.
✓The area range command causes the ABR to advertise only the summary LSA and suppress the more-specific Type-3 LSAs for the range.Correct answer▾
Why this is correct
By default, the area range command summarizes the specified prefix range and suppresses the individual component routes from being advertised into the target area. Only the summary LSA appears in Area 0, which matches the observed behavior.
✗The ABR is filtering the specific /24 routes using a distribute-list under the OSPF process.Wrong answer — click to see why▾
Why this is wrong here
A distribute-list requires explicit configuration; its absence makes this an unsubstantiated guess.
✗The routers in Area 1 are no longer advertising their /24 routes to the ABR because the ABR is in a different area.Wrong answer — click to see why▾
Why this is wrong here
Area border routers receive all LSAs from non-backbone areas; the area boundary does not stop LSA propagation to the ABR itself.
✗The ABR has automatically created a discard route to null0 for the summary, which is dropping all traffic destined to the summarized networks.Wrong answer — click to see why▾
Why this is wrong here
The null0 route is a loop-prevention mechanism, not an absolute traffic blocker; more-specific entries in the routing table take precedence.
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.
Trap categories for this question
Command / output trap
No distribute-list was configured in the scenario. The symptom aligns with the default behavior of the area range command, which suppresses specific routes without any additional filtering.
Scenario analysis trap
No distribute-list was configured in the scenario. The symptom aligns with the default behavior of the area range command, which suppresses specific routes without any additional filtering.
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: The area range command causes the ABR to advertise only the summary LSA and suppress the more-specific Type-3 LSAs for the range. — The area 0 range command on an OSPF ABR creates a summary Type-3 LSA and suppresses the more-specific Type-3 LSAs within that range. This is expected OSPF route summarization behavior; only the summary route is advertised into Area 0, which explains why the /24 routes are missing. The ABR continues to forward traffic properly because it retains specific routes learned from Area 1, so connectivity within the summary should still work under normal conditions. The other answers either describe non-existent configurations or misunderstand the discard route's role.
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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