Question 683 of 2,152

Route Summarization →hardMultiple SelectObjective-mapped

Quick Answer

The answer is routing loops, suboptimal routing, and black holes. These three symptoms directly indicate route summarization problems because a less specific summary route can point to a router that lacks the specific subnet, causing the router to forward traffic into a loop as it tries to find a more precise match. Similarly, when a summary route is advertised toward a network segment where the actual specific subnets are not reachable, traffic is silently dropped into a black hole, and the loss of granularity forces packets along a less optimal path. On the Cisco CCNP ENARSI 300-410 exam, this question tests your ability to distinguish direct symptoms from unrelated issues like high CPU usage or duplicate IP addresses, which are common traps. A useful memory tip is to think of the “three Bs”: Black holes, Bad paths (suboptimal routing), and Broadcast storms avoided—though loops are the third, so remember “Loops, Loss, and Long paths” for the three core symptoms.

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.

Which THREE symptoms indicate that route summarization may be causing routing issues in a network? (Choose THREE.)

Question 1hardmulti select

Review the full routing breakdown →

A
Suboptimal routing paths are observed for certain destinations.
Summarization can cause routers to choose a less specific route, leading to suboptimal paths.
B
Traffic to some subnets is dropped (black hole) even though the summary route exists.
If the summary route is advertised but the specific subnet is not reachable, traffic may be dropped.
C
Routing loops occur due to less specific summary routes pointing to routers that lack the specific subnet.
This can happen if the summary route is not properly configured to cover all subnets.
D
High CPU utilization on routers due to frequent SPF calculations.
Why wrong: Summarization typically reduces SPF calculations, not increases them.
E
Duplicate IP addresses are detected in the network.
Why wrong: Duplicate IP addresses are a configuration error unrelated to route summarization.

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

✓

Suboptimal routing paths are observed for certain destinations.

Route summarization can cause suboptimal routing because the summary route may point to a less specific path. It can also cause black holes if the summary route is advertised but the specific subnets are not reachable. Additionally, summarization can hide more specific routes, leading to routing loops if the summary route is less specific and points to a router that does not have the specific subnet. High CPU usage is not a direct symptom of summarization issues, and duplicate IP addresses are unrelated.

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.

✓
Suboptimal routing paths are observed for certain destinations.
Why this is correct
Summarization can cause routers to choose a less specific route, leading to suboptimal paths.
Related concept
CIDR notation defines the prefix length.
✓
Traffic to some subnets is dropped (black hole) even though the summary route exists.
Why this is correct
If the summary route is advertised but the specific subnet is not reachable, traffic may be dropped.
Related concept
CIDR notation defines the prefix length.
✓
Routing loops occur due to less specific summary routes pointing to routers that lack the specific subnet.
Why this is correct
This can happen if the summary route is not properly configured to cover all subnets.
Related concept
CIDR notation defines the prefix length.
✗
High CPU utilization on routers due to frequent SPF calculations.
Why it's wrong here
Summarization typically reduces SPF calculations, not increases them.
✗
Duplicate IP addresses are detected in the network.
Why it's wrong here
Duplicate IP addresses are a configuration error unrelated to route summarization.

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 small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.

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: Suboptimal routing paths are observed for certain destinations. — Route summarization can cause suboptimal routing because the summary route may point to a less specific path. It can also cause black holes if the summary route is advertised but the specific subnets are not reachable. Additionally, summarization can hide more specific routes, leading to routing loops if the summary route is less specific and points to a router that does not have the specific subnet. High CPU usage is not a direct symptom of summarization issues, and duplicate IP addresses are unrelated.

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

What is the key concept behind this question?

CIDR notation defines the prefix length.

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