Question 908 of 2,152

Route Summarization →mediumMultiple ChoiceObjective-mapped

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 EIGRP network where route summarization is configured. Router R1 has the 'ip summary-address eigrp 100 10.0.0.0 255.0.0.0' command on its interface facing R2. After the configuration, R2 loses connectivity to the 10.1.0.0/16 subnet, which is one of the component routes. The engineer checks the routing table on R2 and sees the summary route 10.0.0.0/8 but not the specific route. 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.

Question 1mediummultiple choice

Study the full EIGRP explanation →

A
The summary route 10.0.0.0/8 is being advertised with a metric of infinity, causing it to be ignored.
Why wrong: Incorrect. A metric of infinity would prevent the route from being installed, but the specific routes would still be suppressed.
B
The 10.1.0.0/16 subnet is not directly connected to R1, so it cannot be summarized.
Why wrong: Incorrect. EIGRP can summarize routes learned from any interface.
C
The summary address command was applied on the wrong interface, causing the summary to be sent out all interfaces, including the one facing the 10.1.0.0/16 subnet's origin.
Why wrong: Incorrect. The summary is applied on a specific interface; it does not affect other interfaces.
D
The 10.1.0.0/16 subnet is not included in the summary range because the summary mask is /8, but the subnet's network address is 10.1.0.0, which is within the range, but the EIGRP process may have a split-horizon issue or the component route is not in the EIGRP topology table.
Correct. If the component route is not in the EIGRP topology table (e.g., due to a missing network statement or a passive interface), the summary route may still be generated, but the specific route is not advertised, causing loss of connectivity.

Full breakdown with real-world context →

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 10.1.0.0/16 subnet is not included in the summary range because the summary mask is /8, but the subnet's network address is 10.1.0.0, which is within the range, but the EIGRP process may have a split-horizon issue or the component route is not in the EIGRP topology table.

In EIGRP, the summary address command suppresses the advertisement of more specific routes and generates the summary. However, if the summary route is not installed in the routing table (e.g., due to a missing component), the specific routes may still be suppressed, causing a black hole.

Key principle: Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.

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 10.0.0.0/8 is being advertised with a metric of infinity, causing it to be ignored.
Why it's wrong here
Incorrect. A metric of infinity would prevent the route from being installed, but the specific routes would still be suppressed.
✗
The 10.1.0.0/16 subnet is not directly connected to R1, so it cannot be summarized.
Why it's wrong here
Incorrect. EIGRP can summarize routes learned from any interface.
✗
The summary address command was applied on the wrong interface, causing the summary to be sent out all interfaces, including the one facing the 10.1.0.0/16 subnet's origin.
Why it's wrong here
Incorrect. The summary is applied on a specific interface; it does not affect other interfaces.
✓
The 10.1.0.0/16 subnet is not included in the summary range because the summary mask is /8, but the subnet's network address is 10.1.0.0, which is within the range, but the EIGRP process may have a split-horizon issue or the component route is not in the EIGRP topology table.
Why this is correct
Correct. If the component route is not in the EIGRP topology table (e.g., due to a missing network statement or a passive interface), the summary route may still be generated, but the specific route is not advertised, causing loss of connectivity.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
CIDR notation defines the prefix length.

Common exam traps

Common exam trap: usable hosts are not the same as total addresses

Subnetting questions often tempt you into counting all addresses. In normal IPv4 subnets, the network and broadcast addresses are not usable host addresses.

Detailed technical explanation

How to think about this question

Subnetting questions test whether you can identify the network, broadcast address, usable range, mask and correct subnet. Slow down enough to calculate the block size correctly.

KKey Concepts to Remember

CIDR notation defines the prefix length.
Block size helps identify subnet boundaries.
Network and broadcast addresses are not usable hosts in normal IPv4 subnets.
The required host count determines the smallest suitable subnet.

TExam Day Tips

Write the block size before choosing the subnet.
Check whether the question asks for hosts, subnets or a specific address range.
Do not confuse /24, /25, /26 and /27 host counts.

Key takeaway

Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.

Real-world example

How this comes up in practice

A network engineer segments a warehouse floor into three subnets: 20 scanners, 5 printers, and 2 management hosts. Picking the wrong mask wastes addresses or leaves too few usable hosts. Exam questions test whether you can apply CIDR notation, calculate block size, and identify the correct usable-host range for a given prefix.

What to study next

Got this wrong? Here's your next step.

Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related 300-410 subnetting questions on CIDR, address ranges, and subnet selection.

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FAQ

Questions learners often ask

What does this 300-410 question test?

Route Summarization — This question tests Route Summarization — CIDR notation defines the prefix length..

What is the correct answer to this question?

The correct answer is: The 10.1.0.0/16 subnet is not included in the summary range because the summary mask is /8, but the subnet's network address is 10.1.0.0, which is within the range, but the EIGRP process may have a split-horizon issue or the component route is not in the EIGRP topology table. — In EIGRP, the summary address command suppresses the advertisement of more specific routes and generates the summary. However, if the summary route is not installed in the routing table (e.g., due to a missing component), the specific routes may still be suppressed, causing a black hole.

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

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?

CIDR notation defines the prefix length.

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