- A
The summary route 192.168.0.0/22 is being advertised with a metric of 0, causing R3 to prefer a less specific route from another source.
Why wrong: Incorrect. A metric of 0 is valid; it would not cause loss of connectivity to one specific subnet.
- B
The summary-address command was applied on R1 as an ABR, but R1 is not an ABR because it is in area 0 only; the command is ignored.
Why wrong: Incorrect. The summary-address command on an ABR is valid for summarizing routes from other areas, but if R1 is not an ABR, the command may not work as expected, but this would affect all subnets.
- C
The 192.168.0.0/24 subnet is not included in the summary because the summary mask is /22, but the subnet mask of 192.168.0.0/24 is not contiguous with the others due to a configuration error.
Why wrong: Incorrect. 192.168.0.0/24 is within the /22 range, so it should be included.
- D
The summary-address command on R1 is configured with the 'not-advertise' keyword, suppressing the summary but not the specific routes, causing a routing black hole for the 192.168.0.0/24 subnet.
Correct. The 'not-advertise' keyword in OSPF summary-address prevents the summary from being advertised, but the specific routes may still be suppressed if the command is misapplied, leading to loss of connectivity to the specific subnet.
Quick Answer
The answer is the summary-address command on R1 configured with the 'not-advertise' keyword. This keyword suppresses the summary route itself while still allowing the specific /24 subnets to be advertised individually, but because the summary is not injected into the OSPF database, R3 lacks a route for 192.168.0.0/24 when the specific route for that subnet is also inadvertently suppressed or not installed, creating a routing black hole. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding of how OSPF route summarization interacts with the not-advertise option—a common trap is assuming that suppressing the summary also suppresses all specific routes, when in fact it only blocks the aggregate. Remember the memory tip: "Not-advertise kills the parent, not the children."
300-410 Route Summarization Practice Question
This 300-410 practice question tests your understanding of route summarization. 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.
A network engineer is troubleshooting an OSPF network where routers R1, R2, and R3 are in area 0. R1 has a summary route 192.168.0.0/22 configured on its interface to R2, summarizing four /24 subnets (192.168.0.0/24 through 192.168.3.0/24). After the configuration, R3 loses connectivity to the 192.168.0.0/24 subnet, although other subnets are reachable. 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 summary-address command on R1 is configured with the 'not-advertise' keyword, suppressing the summary but not the specific routes, causing a routing black hole for the 192.168.0.0/24 subnet.
The issue is that the summary route may be suppressing the more specific routes, but the summary itself might not be installed due to a mismatch or missing component. In OSPF, the summary-address command on an ASBR or ABR can cause issues if the metric or type is misconfigured.
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 summary route 192.168.0.0/22 is being advertised with a metric of 0, causing R3 to prefer a less specific route from another source.
Why it's wrong here
Incorrect. A metric of 0 is valid; it would not cause loss of connectivity to one specific subnet.
- ✗
The summary-address command was applied on R1 as an ABR, but R1 is not an ABR because it is in area 0 only; the command is ignored.
Why it's wrong here
Incorrect. The summary-address command on an ABR is valid for summarizing routes from other areas, but if R1 is not an ABR, the command may not work as expected, but this would affect all subnets.
- ✗
The 192.168.0.0/24 subnet is not included in the summary because the summary mask is /22, but the subnet mask of 192.168.0.0/24 is not contiguous with the others due to a configuration error.
Why it's wrong here
Incorrect. 192.168.0.0/24 is within the /22 range, so it should be included.
- ✓
The summary-address command on R1 is configured with the 'not-advertise' keyword, suppressing the summary but not the specific routes, causing a routing black hole for the 192.168.0.0/24 subnet.
Why this is correct
Correct. The 'not-advertise' keyword in OSPF summary-address prevents the summary from being advertised, but the specific routes may still be suppressed if the command is misapplied, leading to loss of connectivity to the specific subnet.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
OSPF neighbours must agree on key parameters.
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
Incorrect. The summary-address command on an ABR is valid for summarizing routes from other areas, but if R1 is not an ABR, the command may not work as expected, but this would affect all subnets.
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 300-410 OSPF questions on adjacency and route selection.
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FAQ
Questions learners often ask
What does this 300-410 question test?
Route Summarization — This question tests Route Summarization — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: The summary-address command on R1 is configured with the 'not-advertise' keyword, suppressing the summary but not the specific routes, causing a routing black hole for the 192.168.0.0/24 subnet. — The issue is that the summary route may be suppressing the more specific routes, but the summary itself might not be installed due to a mismatch or missing component. In OSPF, the summary-address command on an ASBR or ABR can cause issues if the metric or type is misconfigured.
What should I do if I get this 300-410 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 300-410 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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Same concept, more angles
1 more ways this is tested on 300-410
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. A network engineer is troubleshooting a redistribution issue between EIGRP and OSPF. Router R1 redistributes EIGRP routes into OSPF. The engineer configured a summary route 10.0.0.0/8 using the 'summary-address' command under the OSPF process. After the configuration, OSPF neighbors lose connectivity to the 10.1.0.0/16 subnet, which is one of the component routes. What is the most likely cause?
hard- A.The summary-address command on R1 is configured with the 'tag' keyword, causing the summary to be ignored by other routers.
- ✓ B.The summary route 10.0.0.0/8 is not being generated because the component routes are not all present in the OSPF database.
- C.The OSPF neighbor relationship is down due to a mismatch in area IDs.
- D.The engineer forgot to configure the 'network' command for the summary route under OSPF.
Why B: The issue is that the summary-address command in OSPF can suppress the advertisement of more specific routes, but if the summary route is not installed due to a missing component or metric issue, it can cause a routing black hole.
Last reviewed: Jun 18, 2026
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