Question 42 of 2,152

Route Summarization →mediumMultiple ChoiceObjective-mapped

Quick Answer

The answer is a split-horizon issue caused by the RIP summary route being generated on the same interface from which the specific 192.168.1.0/24 route was learned. When you configure the `ip summary-address rip 192.168.1.0 255.255.252.0` command, RIP suppresses the advertisement of more specific subnets within that /22 range, but split-horizon prevents the summary from being sent back out the interface where the specific route was originally received. This creates a scenario where R2 loses the 192.168.1.0/24 route because the summary is blocked, yet the specific route is also suppressed. On the Cisco CCNP ENARSI 300-410 exam, this tests your understanding of how RIP route summarization interacts with split-horizon, a common trap where engineers assume the summary automatically covers all subnets. The key memory tip: “Summarize, but don’t split—if the source is the same interface, the route won’t hit.”

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 a route summarization issue in a network using RIP. Router R1 is configured with the 'ip summary-address rip 192.168.0.0 255.255.252.0' command on its serial interface. After the configuration, R2, which is connected via that interface, can no longer reach the 192.168.1.0/24 subnet, although other subnets within the summary 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.

Question 1mediummultiple choice

Review the full subnetting walkthrough →

A
The 192.168.1.0/24 subnet is not directly connected to R1, so it cannot be summarized.
Why wrong: Incorrect. RIP can summarize routes learned from other interfaces as well.
B
The summary route 192.168.0.0/22 is being advertised with a higher metric than the specific routes, causing R2 to prefer a different path.
Why wrong: Incorrect. The summary route would replace the specific routes, not compete with them.
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 192.168.1.0/24 subnet's origin.
Why wrong: Incorrect. The summary is applied on a specific interface; it does not affect other interfaces.
D
The 192.168.1.0/24 subnet is not included in the summary range because the summary mask is /22, but the subnet's network address is 192.168.1.0, which is within the range, but the RIP process may have a split-horizon issue preventing the route from being advertised.
Correct. In RIP, split horizon prevents a route from being advertised out the interface it was learned on. If the 192.168.1.0/24 subnet was learned on the same interface where the summary is applied, it will not be 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 192.168.1.0/24 subnet is not included in the summary range because the summary mask is /22, but the subnet's network address is 192.168.1.0, which is within the range, but the RIP process may have a split-horizon issue preventing the route from being advertised.

In RIP, the summary-address command suppresses the advertisement of more specific routes. However, if the summary route is not installed in the routing table (e.g., due to a metric issue), the specific routes may still be suppressed, causing loss of connectivity.

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 192.168.1.0/24 subnet is not directly connected to R1, so it cannot be summarized.
Why it's wrong here
Incorrect. RIP can summarize routes learned from other interfaces as well.
✗
The summary route 192.168.0.0/22 is being advertised with a higher metric than the specific routes, causing R2 to prefer a different path.
Why it's wrong here
Incorrect. The summary route would replace the specific routes, not compete with them.
✗
The summary address command was applied on the wrong interface, causing the summary to be sent out all interfaces, including the one facing the 192.168.1.0/24 subnet's origin.
Why it's wrong here
Incorrect. The summary is applied on a specific interface; it does not affect other interfaces.
✓
The 192.168.1.0/24 subnet is not included in the summary range because the summary mask is /22, but the subnet's network address is 192.168.1.0, which is within the range, but the RIP process may have a split-horizon issue preventing the route from being advertised.
Why this is correct
Correct. In RIP, split horizon prevents a route from being advertised out the interface it was learned on. If the 192.168.1.0/24 subnet was learned on the same interface where the summary is applied, it will not be 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 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: The 192.168.1.0/24 subnet is not included in the summary range because the summary mask is /22, but the subnet's network address is 192.168.1.0, which is within the range, but the RIP process may have a split-horizon issue preventing the route from being advertised. — In RIP, the summary-address command suppresses the advertisement of more specific routes. However, if the summary route is not installed in the routing table (e.g., due to a metric issue), the specific routes may still be suppressed, causing loss of connectivity.

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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