- A
The ACL is blocking OSPF hello packets, preventing the neighbor from reaching FULL state.
An ACL denying protocol 89 blocks all OSPF packets, including hellos and DBDs, causing the neighbor to remain in EXSTART as DBD exchange fails.
- B
The MTU on the interface is mismatched with the neighbor, causing DBD packets to be fragmented.
Why wrong: While MTU mismatch can cause EXSTART, the ACL is the more direct issue here as it explicitly denies OSPF.
- C
The ACL is applied outbound instead of inbound, filtering OSPF updates.
Why wrong: Outbound ACL would block outgoing OSPF packets, but the symptom of stuck in EXSTART with hellos exchanged suggests inbound filtering.
- D
The router has a distribute-list in OSPF that is filtering routes.
Why wrong: Distribute-list filters routes in the routing table, not OSPF adjacency packets, so it would not cause EXSTART.
Quick Answer
The answer is that the ACL is blocking OSPF hello packets, which prevents the neighbor from reaching the FULL state. When an ACL denies OSPF protocol (IP protocol 89) inbound on a serial interface, it silently drops all OSPF packet types, including the Hello packets required for neighbor discovery and maintenance. Without these Hellos, the routers can form a basic adjacency and reach EXSTART, but they cannot confirm bidirectional communication or exchange Database Description packets, leaving them stuck in EXSTART rather than progressing to FULL. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding of how ACL filtering interacts with OSPF state machine transitions—a common trap is assuming that only unicast traffic is blocked, when in fact protocol-based ACLs filter all packets of that type, including multicast Hellos. Remember the memory tip: “EXSTART means Hellos are apart”—if Hellos are denied, the adjacency stalls at EXSTART because the routers cannot complete the DBD exchange needed to move forward.
300-410 IPv4 Access Control Lists Practice Question
This 300-410 practice question tests your understanding of ipv4 access control lists. 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 engineer configures an IPv4 ACL with a deny statement for OSPF protocol (IP protocol 89) on a router's serial interface inbound. OSPF neighbors are established, but routes are not exchanged and the neighbor state remains in EXSTART. 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 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 ACL is blocking OSPF hello packets, preventing the neighbor from reaching FULL state.
When an ACL denies OSPF (IP protocol 89) inbound on a serial interface, it blocks all OSPF packets, including Hello packets. Without Hello packets, the neighbor cannot transition from EXSTART to FULL state because the OSPF adjacency process requires Hello exchange for neighbor discovery and maintenance. The EXSTART state indicates that the routers have formed a neighbor relationship but cannot exchange Database Description (DBD) packets, which occurs because the ACL is filtering the Hello packets that are needed to confirm bidirectional communication and proceed to the Exchange state.
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 is blocking OSPF hello packets, preventing the neighbor from reaching FULL state.
Why this is correct
An ACL denying protocol 89 blocks all OSPF packets, including hellos and DBDs, causing the neighbor to remain in EXSTART as DBD exchange fails.
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 MTU on the interface is mismatched with the neighbor, causing DBD packets to be fragmented.
- ✗
The ACL is applied outbound instead of inbound, filtering OSPF updates.
- ✗
The router has a distribute-list in OSPF that is filtering routes.
Why it's wrong here
Distribute-list filters routes in the routing table, not OSPF adjacency packets, so it would not cause EXSTART.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that an ACL blocking OSPF protocol 89 only affects route updates, when in fact it blocks all OSPF packets including Hello, which prevents the adjacency from reaching FULL state.
Detailed technical explanation
How to think about this question
OSPF uses IP protocol 89 for all its packet types (Hello, DBD, LSR, LSU, LSAck). An ACL denying protocol 89 blocks all OSPF traffic, not just specific packet types. The EXSTART state is the first step in the database exchange process where the master/slave relationship is negotiated using DBD packets; if Hello packets are blocked, the neighbor cannot maintain the adjacency and will not proceed to Exchange state, but the router may still show EXSTART because the DBD exchange never completes due to missing Hello packets that confirm the neighbor is still reachable.
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.
What to study next
Got this wrong? Here's your next step.
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FAQ
Questions learners often ask
What does this 300-410 question test?
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 is blocking OSPF hello packets, preventing the neighbor from reaching FULL state. — When an ACL denies OSPF (IP protocol 89) inbound on a serial interface, it blocks all OSPF packets, including Hello packets. Without Hello packets, the neighbor cannot transition from EXSTART to FULL state because the OSPF adjacency process requires Hello exchange for neighbor discovery and maintenance. The EXSTART state indicates that the routers have formed a neighbor relationship but cannot exchange Database Description (DBD) packets, which occurs because the ACL is filtering the Hello packets that are needed to confirm bidirectional communication and proceed to the Exchange state.
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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Last reviewed: Jun 24, 2026
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.
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