- A
The EIGRP summary address 10.10.0.0/22 is configured on the wrong interface; it should be on the interface facing the remote site.
Why wrong: The summary is correctly placed on the interface toward R2, but the issue is that the summary suppresses the specific route without providing a valid path.
- B
The EIGRP summary address 10.10.0.0/22 is causing R1 to suppress the advertisement of 10.10.10.0/24, and R2 does not have a route to the summary because the summary route is not installed in R1's routing table.
The summary suppresses the specific route, but if the summary is not installed (e.g., no discard route), R2 never learns any route to 10.10.10.0/24.
- C
Split horizon is enabled on R1's GigabitEthernet0/0, preventing the advertisement of 10.10.10.0/24 learned from another interface.
Why wrong: Split horizon would block routes learned from the same interface, but 10.10.10.0/24 is likely connected or learned from another interface, so split horizon is not the primary issue.
- D
The EIGRP metric for 10.10.10.0/24 is too high, causing it to be suppressed by the summary route.
Why wrong: EIGRP summary routes do not suppress based on metric; they suppress all more specific routes regardless of metric.
Quick Answer
The root cause is that the EIGRP summary route 10.10.0.0/22 configured on R1 is suppressing the advertisement of the more-specific subnet 10.10.10.0/24, yet the summary itself is not installed in R1’s routing table, so R2 never receives any route to that subnet. This happens because the `ip summary-address eigrp` command creates a local summary that blocks all component routes from being advertised out the interface, but the summary route only appears in the routing table if a matching component route is already present—if the specific /24 is learned via another interface or is absent, the summary is omitted entirely. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding of EIGRP summary route suppression and the critical distinction between the topology table and the routing table; a common trap is assuming the summary is always installed. Remember the memory tip: “Summary suppresses specifics, but needs a component to appear in the table.”
300-410 Device Access Control Practice Question
This 300-410 practice question tests your understanding of device access control. Examine the command output carefully: the correct answer depends on what the output actually shows, not on general recall alone. 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 large enterprise network is experiencing intermittent loss of reachability to a critical subnet 10.10.10.0/24 from remote sites. Router R1 has the following relevant configuration: interface GigabitEthernet0/0 ip address 192.168.1.1 255.255.255.0 ip summary-address eigrp 100 10.10.0.0 255.255.252.0. Router R2 shows: show ip route eigrp | include 10.10.10.0 outputs nothing, but show ip eigrp topology all-links shows 10.10.10.0/24 via 192.168.1.1 with a feasible distance of 1280. What is the root cause?
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 EIGRP summary address 10.10.0.0/22 is causing R1 to suppress the advertisement of 10.10.10.0/24, and R2 does not have a route to the summary because the summary route is not installed in R1's routing table.
The ip summary-address eigrp 100 10.10.0.0 255.255.252.0 command on R1's GigabitEthernet0/0 creates a local summary route that suppresses all more-specific routes within the 10.10.0.0/22 range, including 10.10.10.0/24, from being advertised out that interface. However, the summary route itself is not installed in R1's routing table unless a component route exists in the table, which may not be the case if the specific route is learned via another interface or is not present. As a result, R2 never receives either the specific /24 or the summary /22, leading to the intermittent loss of reachability.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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 EIGRP summary address 10.10.0.0/22 is configured on the wrong interface; it should be on the interface facing the remote site.
Why it's wrong here
The summary is correctly placed on the interface toward R2, but the issue is that the summary suppresses the specific route without providing a valid path.
- ✓
The EIGRP summary address 10.10.0.0/22 is causing R1 to suppress the advertisement of 10.10.10.0/24, and R2 does not have a route to the summary because the summary route is not installed in R1's routing table.
Why this is correct
The summary suppresses the specific route, but if the summary is not installed (e.g., no discard route), R2 never learns any route to 10.10.10.0/24.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
Split horizon is enabled on R1's GigabitEthernet0/0, preventing the advertisement of 10.10.10.0/24 learned from another interface.
Why it's wrong here
Split horizon would block routes learned from the same interface, but 10.10.10.0/24 is likely connected or learned from another interface, so split horizon is not the primary issue.
- ✗
The EIGRP metric for 10.10.10.0/24 is too high, causing it to be suppressed by the summary route.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the nuance that EIGRP summary routes suppress more-specific routes but are only advertised if a component route exists in the routing table, leading candidates to incorrectly assume the summary is always advertised or that the issue is with split horizon or interface placement.
Detailed technical explanation
How to think about this question
When EIGRP is configured with a summary address, it installs a local discard route (null0) for the summary prefix in the routing table to prevent loops, but the summary is only advertised to neighbors if at least one component route is present in the topology table. In this scenario, the 10.10.10.0/24 route is in the topology table with a feasible distance of 1280, but if R1's routing table does not contain a valid successor for that route (e.g., due to a flapping link or passive interface), the summary route may not be advertised, leaving R2 without any route to the subnet. This behavior is often overlooked in designs where summarization is used on interfaces that should propagate the summary even without a component route in the table.
KKey Concepts to Remember
- Read the scenario before looking for a memorised answer.
- Find the constraint that changes the correct option.
- Eliminate answers that are true in general but not in this case.
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
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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FAQ
Questions learners often ask
What does this 300-410 question test?
Device Access Control — This question tests Device Access Control — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The EIGRP summary address 10.10.0.0/22 is causing R1 to suppress the advertisement of 10.10.10.0/24, and R2 does not have a route to the summary because the summary route is not installed in R1's routing table. — The ip summary-address eigrp 100 10.10.0.0 255.255.252.0 command on R1's GigabitEthernet0/0 creates a local summary route that suppresses all more-specific routes within the 10.10.0.0/22 range, including 10.10.10.0/24, from being advertised out that interface. However, the summary route itself is not installed in R1's routing table unless a component route exists in the table, which may not be the case if the specific route is learned via another interface or is not present. As a result, R2 never receives either the specific /24 or the summary /22, leading to the intermittent loss of reachability.
What should I do if I get this 300-410 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 24, 2026
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