Question 231 of 2,152

IPv4 Access Control Lists →hardMultiple ChoiceObjective-mapped

ACL Denying OSPF Protocol

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 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.

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.

Full breakdown with real-world context →

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.

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.

The ACL denies all OSPF packets (IP protocol 89) inbound on the serial interface. The EXSTART state indicates that the router has already exchanged Hello packets with the neighbor and formed a neighbor relationship. However, the ACL blocks Database Description (DBD) packets, which are required for transitioning from EXSTART to FULL state. Without successful DBD exchange, the adjacency remains stuck in EXSTART. Although the ACL also blocks Hello packets, the neighbor relationship was already established before the ACL took effect or Hello packets are not the immediate cause of the EXSTART state. This question tests the understanding that blocking OSPF affects not only route updates but also DBD packets necessary for database synchronization.

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.
Why it's wrong here
While MTU mismatch can cause EXSTART, the ACL is the more direct issue here as it explicitly denies OSPF.
✗
The ACL is applied outbound instead of inbound, filtering OSPF updates.
Why it's wrong here
Outbound ACL would block outgoing OSPF packets, but the symptom of stuck in EXSTART with hellos exchanged suggests inbound filtering.
✗
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 DBD packets. In this scenario, the adjacency reaches EXSTART (because Hello succeeded) but cannot progress further because DBD packets are filtered.

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.

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

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. — The ACL denies all OSPF packets (IP protocol 89) inbound on the serial interface. The EXSTART state indicates that the router has already exchanged Hello packets with the neighbor and formed a neighbor relationship. However, the ACL blocks Database Description (DBD) packets, which are required for transitioning from EXSTART to FULL state. Without successful DBD exchange, the adjacency remains stuck in EXSTART. Although the ACL also blocks Hello packets, the neighbor relationship was already established before the ACL took effect or Hello packets are not the immediate cause of the EXSTART state. This question tests the understanding that blocking OSPF affects not only route updates but also DBD packets necessary for database synchronization.

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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