The answer is that the inbound ACL blocks OSPF updates, causing the router to prefer the EIGRP-learned route due to its lower administrative distance. When an ACL denies OSPF-learned routes on the interface, those prefixes are not installed in the routing table, but mutual redistribution ensures the same prefix is still learned via EIGRP. Since EIGRP’s default AD of 90 is lower than OSPF’s 110, the router selects the EIGRP path, and the prefix remains in the table with a different next hop. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding of how ACL blocking OSPF updates interacts with route selection via redistribution—a common trap is assuming the prefix disappears entirely rather than switching protocols. Remember the memory tip: “ACL blocks the update, not the route; lower AD wins the dispute.”
300-410 IPv4 Access Control Lists Practice Question
This 300-410 practice question tests your understanding of ipv4 access control lists. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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 engineer configures mutual redistribution between OSPF and EIGRP on a router. Both protocols have routes for the same prefix. The engineer also applies an inbound ACL on the OSPF interface to deny certain routes from being learned via OSPF. After the ACL is applied, the router still has the prefix in the routing table, but it is learned via EIGRP instead of OSPF. What is the most likely explanation?
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.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The ACL blocks OSPF updates, so the router learns the prefix via EIGRP with a lower AD.
Option A is correct because the inbound ACL on the OSPF interface blocks the OSPF-learned route from being installed in the routing table. Since mutual redistribution is configured, the same prefix is also learned via EIGRP. EIGRP has a default administrative distance (AD) of 90 for internal routes, which is lower than OSPF's AD of 110. Therefore, the router selects the EIGRP route as the best path, and the prefix remains in the routing table but now points to the EIGRP next hop.
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 ACL blocks OSPF updates, so the router learns the prefix via EIGRP with a lower AD.
Why this is correct
The ACL prevents OSPF from learning the route, but redistribution allows EIGRP to carry it, and EIGRP's lower AD makes it the best path.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
The ACL is applied outbound, so it filters OSPF routes being sent to the neighbor.
Why it's wrong here
Outbound ACL would affect outgoing updates, not incoming learning.
✗
The router has a static route for the prefix with AD 1.
Why it's wrong here
Static route would override both, but the question does not mention static.
✗
The OSPF process has a distribute-list that is filtering the route.
Why it's wrong here
Distribute-list would filter the route from the routing table, but the ACL is the configured filter.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the distinction between filtering routing updates via an interface ACL (which blocks the entire OSPF packet, preventing route learning) versus using a distribute-list (which filters specific routes within the routing process), leading candidates to confuse the two mechanisms.
Detailed technical explanation
How to think about this question
When mutual redistribution is configured, the same prefix can be injected into both OSPF and EIGRP, leading to potential routing loops if not managed with route tagging or administrative distance adjustments. The inbound ACL on the OSPF interface uses the `ip access-group` command to filter incoming packets, which can prevent OSPF hello or update packets from being processed, effectively blocking the OSPF route from being learned. In real-world scenarios, this behavior can be used to influence path selection by selectively blocking OSPF-learned routes, forcing the router to rely on EIGRP with its lower AD.
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 at a university connects two campus buildings via a fibre link. Both routers run OSPF, but no adjacency forms — even though both routers can ping each other. The engineer finds one router is in area 0 and the other in area 1. OSPF adjacency requires matching area numbers, hello/dead timers, and network type. IP reachability alone is not enough.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 300-410 question in full detail.
IPv4 Access Control Lists — This question tests IPv4 Access Control Lists — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The ACL blocks OSPF updates, so the router learns the prefix via EIGRP with a lower AD. — Option A is correct because the inbound ACL on the OSPF interface blocks the OSPF-learned route from being installed in the routing table. Since mutual redistribution is configured, the same prefix is also learned via EIGRP. EIGRP has a default administrative distance (AD) of 90 for internal routes, which is lower than OSPF's AD of 110. Therefore, the router selects the EIGRP route as the best path, and the prefix remains in the routing table but now points to the EIGRP next hop.
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.
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?
Read the scenario before looking for a memorised answer.
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