Question 120 of 2,152

Route Summarization →hardMultiple ChoiceObjective-mapped

OSPF summary-address Command: Common Pitfalls and Solutions

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. A key principle to apply: oSPF Internal Summarization. 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.

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: The summary route being advertised with a metric of 0 would not cause a more specific route to be preferred; OSPF always prefers the most specific match, so a /24 route would be preferred over a /22.
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.
Correct. The 'summary-address' command is only valid on ASBRs for external routes. On an internal router in area 0, the command is ignored. Internal summarization requires the 'area range' command on an ABR.
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: The subnet 192.168.0.0/24 is included in the summary 192.168.0.0/22 because the summary covers 192.168.0.0 through 192.168.3.0. There is no contiguity issue with the subnet masks.
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.
Why wrong: The 'not-advertise' keyword in 'summary-address' suppresses only the summary route itself, not the specific component routes. Therefore, it would not create a black hole for the specific /24 subnet.

Full breakdown with real-world context →

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."

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 was applied on R1 as an ABR, but R1 is not an ABR because it is in area 0 only; the command is ignored.

The most likely cause is that the 'summary-address' command was applied on R1, but R1 is in area 0 only and is not an Area Border Router (ABR). The 'summary-address' command is used on an Autonomous System Boundary Router (ASBR) to summarize external routes into OSPF, not for internal summarization. For internal routes, the 'area range' command must be used on an ABR. Since R1 is not an ABR, the command is ignored, and the summarization does not occur. The loss of connectivity to 192.168.0.0/24 is likely due to a different configuration error or a transient issue, but option B correctly identifies the misapplication of the command.

Key principle: OSPF Internal Summarization

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
The summary route being advertised with a metric of 0 would not cause a more specific route to be preferred; OSPF always prefers the most specific match, so a /24 route would be preferred over a /22.
✓
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 this is correct
Correct. The 'summary-address' command is only valid on ASBRs for external routes. On an internal router in area 0, the command is ignored. Internal summarization requires the 'area range' command on an ABR.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
OSPF Internal Summarization
✗
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
The subnet 192.168.0.0/24 is included in the summary 192.168.0.0/22 because the summary covers 192.168.0.0 through 192.168.3.0. There is no contiguity issue with the subnet masks.
✗
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 it's wrong here
The 'not-advertise' keyword in 'summary-address' suppresses only the summary route itself, not the specific component routes. Therefore, it would not create a black hole for the specific /24 subnet.

Common exam traps

Common exam trap: answer the scenario, not the keyword

Candidates often confuse the 'summary-address' command (used on ASBR for external routes) with the 'area range' command (used on ABR for internal routes). In this scenario, R1 is not an ABR, so any attempt to summarize internal routes is ignored.

Trap categories for this question

Keyword trap
The 'not-advertise' keyword in 'summary-address' suppresses only the summary route itself, not the specific component routes. Therefore, it would not create a black hole for the specific /24 subnet.

Detailed technical explanation

How to think about this question

Treat this as a scenario question. Identify the problem, the constraint, and the best action. Then compare each option against those facts.

KKey Concepts to Remember

OSPF Internal Summarization
OSPF External Summarization
ABR (Area Border Router)

TExam Day Tips

Watch for words such as best, first, most likely and least administrative effort.
Review why wrong options are wrong, not only why the correct option is correct.

Key takeaway

OSPF Internal Summarization

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.

Visual reference

Quick reference

Routing Protocol Comparison

Protocol	Metric	Max Hops	Algorithm	Type
RIP v2	Hop count	15	Bellman-Ford	Distance vector
OSPF	Cost (bandwidth)	Unlimited	Dijkstra (SPF)	Link state
EIGRP	Composite metric	Unlimited	DUAL	Hybrid
IS-IS	Cost	Unlimited	Dijkstra	Link state
BGP	Policy / attributes	Unlimited	Path vector	Path vector

RIP's 15-hop limit makes it unsuitable for large networks. OSPF and EIGRP dominate modern enterprise deployments.

What to study next

Got this wrong? Here's your next step.

Review oSPF Internal Summarization, then practise related 300-410 questions on the same topic to reinforce the concept.

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FAQ

Questions learners often ask

What does this 300-410 question test?

Route Summarization — This question tests Route Summarization — OSPF Internal Summarization.

What is the correct answer to this question?

The correct answer is: 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. — The most likely cause is that the 'summary-address' command was applied on R1, but R1 is in area 0 only and is not an Area Border Router (ABR). The 'summary-address' command is used on an Autonomous System Boundary Router (ASBR) to summarize external routes into OSPF, not for internal summarization. For internal routes, the 'area range' command must be used on an ABR. Since R1 is not an ABR, the command is ignored, and the summarization does not occur. The loss of connectivity to 192.168.0.0/24 is likely due to a different configuration error or a transient issue, but option B correctly identifies the misapplication of the command.

What should I do if I get this 300-410 question wrong?

Review oSPF Internal Summarization, then practise related 300-410 questions on the same topic to reinforce the concept.

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 Internal Summarization

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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.

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