hardMultiple Choice
300-410 Practice Question: An enterprise network uses EIGRP with route…
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?
⚠ Common exam trap
Test-takers frequently 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.
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.
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.
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
A missing null0 route does not prevent EIGRP from advertising a configured summary address. The null0 route is only used locally to prevent routing loops when a summary is advertised; without it, the router may still send the summary to neighbors but could have suboptimal forwarding or loops within its own routing table. Therefore, the issue described cannot be attributed to the absence of a null0 route.
- ✗
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
The summary address is meant to be configured on the interface through which the summary is advertised to downstream neighbors, which is typically the upstream interface facing R2, not the interface facing the internal network. If configured on the internal interface, EIGRP would summarize routes out that interface, but the summary would not be sent toward R2 as intended. Thus, placing it on the interface facing the internal network would not cause the specific route to be suppressed on R2's neighbor relationship.
- ✓
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
Configuring ip summary-address eigrp on R1's interface toward R2 creates a summary route 10.1.0.0/22 that covers the 10.1.1.0/24 subnet. EIGRP automatically suppresses advertisement of any more specific routes that fall within the summary, so R2 receives only the summary and not the /24. This behavior is normal and expected; however, if the summary does not accurately represent the actual networks (e.g., it includes subnets that are not reachable), reachability to those specific prefixes can fail while the summary route still appears in R2's routing table.
- ✗
R2 has a route filter that blocks the 10.1.1.0/24 route.
Why it's wrong here
There is no evidence in the scenario that R2 has a route filter or distribute-list that would block the 10.1.1.0/24 route. Even if R2 had a filter blocking the specific route, it would still receive the 10.1.0.0/22 summary from R1, and the question's issue is about the missing /24 route, not about both routes being absent. The root cause is R1's summary suppressing the specific route, not any filtering on R2.
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.
Go deeper
Related to this question
About these practice questions
One of 1,401 original 300-410 practice questions on Courseiva, each with a full explanation and wrong-answer analysis — not exam dumps or protected exam content. Learn why practice questions differ from exam dumps →
JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 300-410 practice question is part of Courseiva's free Cisco certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 300-410 exam.