- A
R1 has a missing null0 route for the summary, causing it to not advertise the summary correctly.
Why wrong: While a null0 route is recommended for loop prevention, its absence does not prevent the summary from being advertised; it only affects local forwarding.
- B
The summary address 10.1.0.0/22 is configured on the wrong interface; it should be on the interface facing the internal network.
Why wrong: Summarization should be configured on the interface where the summary is advertised, typically the upstream interface, not the internal one.
- C
R1's summary address 10.1.0.0/22 includes the 10.1.1.0/24 subnet, so R1 suppresses the more specific route, and R2 only learns the summary. This is expected behavior, but if the summary does not match the actual network, reachability fails.
EIGRP automatically suppresses more specific routes when a summary is configured. If the summary is correct, this is normal; however, if the summary is too broad or incorrect, specific subnets may be unreachable.
- D
R2 has a route filter that blocks the 10.1.1.0/24 route.
Why wrong: There is no indication of a route filter; the issue is the summary suppressing the specific route.
EIGRP Summary Address Suppression
This 300-410 practice question tests your understanding of eigrp troubleshooting. 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.
An enterprise network uses EIGRP with route summarization. Router R1 has the following configuration: interface GigabitEthernet0/0 ip summary-address eigrp 100 10.1.0.0 255.255.252.0. Router R2, connected to R1 via GigabitEthernet0/0, shows: 'show ip route eigrp' includes 10.1.0.0/22 but not the more specific route 10.1.1.0/24. Hosts in subnet 10.1.1.0/24 are unreachable from R2. What is the root cause?
Quick Answer
The answer is that R1’s summary address 10.1.0.0/22 is suppressing the more specific 10.1.1.0/24 route, causing suboptimal routing and unreachability. This happens because EIGRP summarization on an interface automatically suppresses all component routes that fall within the summary range, so R2 learns only the aggregate and loses the specific path to 10.1.1.0/24. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding of EIGRP summary address suppression and the critical need for a matching null0 route on the summarizing router to prevent black holes. A common trap is assuming the summary will always improve reachability, but without verifying that the summary prefix exactly matches the actual network topology, you can inadvertently drop traffic to specific subnets. Memory tip: “Summary suppresses specifics—if the summary is too broad, you’ve built a road to nowhere.”
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
R1's summary address 10.1.0.0/22 includes the 10.1.1.0/24 subnet, so R1 suppresses the more specific route, and R2 only learns the summary. This is expected behavior, but if the summary does not match the actual network, reachability fails.
Option C is correct because EIGRP's route summarization behavior is to suppress more specific routes (e.g., 10.1.1.0/24) when a covering summary (10.1.0.0/22) is configured on an interface. R2 learns only the summary route, but if the actual network 10.1.1.0/24 is not reachable via the summary's next hop (e.g., because the summary includes subnets not actually present or because R1 lacks a discard route), R2 cannot reach those hosts. The root cause is that the summary route does not accurately represent the reachable subnets, leading to black-holing.
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.
- ✗
R1 has a missing null0 route for the summary, causing it to not advertise the summary correctly.
Why it's wrong here
While a null0 route is recommended for loop prevention, its absence does not prevent the summary from being advertised; it only affects local forwarding.
- ✗
The summary address 10.1.0.0/22 is configured on the wrong interface; it should be on the interface facing the internal network.
Why it's wrong here
Summarization should be configured on the interface where the summary is advertised, typically the upstream interface, not the internal one.
- ✓
R1's summary address 10.1.0.0/22 includes the 10.1.1.0/24 subnet, so R1 suppresses the more specific route, and R2 only learns the summary. This is expected behavior, but if the summary does not match the actual network, reachability fails.
Why this is correct
EIGRP automatically suppresses more specific routes when a summary is configured. If the summary is correct, this is normal; however, if the summary is too broad or incorrect, specific subnets may be unreachable.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
R2 has a route filter that blocks the 10.1.1.0/24 route.
Why it's wrong here
There is no indication of a route filter; the issue is the summary suppressing the specific route.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often assume EIGRP summarization always works correctly once configured, failing to realize that the summary route must exactly match the set of reachable subnets; otherwise, the suppressed specifics lead to unreachability, not a configuration error on the summary command itself.
Detailed technical explanation
How to think about this question
EIGRP route summarization works by advertising a single aggregate route (e.g., 10.1.0.0/22) out the configured interface while suppressing all more specific component routes (e.g., 10.1.1.0/24, 10.1.2.0/24) that fall within the summary range. For the summary to be valid, R1 must have a discard route (automatically installed as a null0 route) to prevent routing loops; if the summary includes subnets that do not exist, traffic destined to those missing subnets will be dropped. In real-world scenarios, misconfigured summarization can cause partial reachability, as seen here, where hosts in a supposedly included subnet are unreachable because the summary does not accurately reflect the actual network topology.
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.
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
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FAQ
Questions learners often ask
What does this 300-410 question test?
EIGRP Troubleshooting — This question tests EIGRP Troubleshooting — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: R1's summary address 10.1.0.0/22 includes the 10.1.1.0/24 subnet, so R1 suppresses the more specific route, and R2 only learns the summary. This is expected behavior, but if the summary does not match the actual network, reachability fails. — Option C is correct because EIGRP's route summarization behavior is to suppress more specific routes (e.g., 10.1.1.0/24) when a covering summary (10.1.0.0/22) is configured on an interface. R2 learns only the summary route, but if the actual network 10.1.1.0/24 is not reachable via the summary's next hop (e.g., because the summary includes subnets not actually present or because R1 lacks a discard route), R2 cannot reach those hosts. The root cause is that the summary route does not accurately represent the reachable subnets, leading to black-holing.
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: Jul 4, 2026
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