Question 716 of 2,152

NetFlow and Flexible NetFlow →hardMultiple ChoiceObjective-mapped

Quick Answer

The answer is that the router with the larger MTU sends DBD packets that exceed the smaller MTU, causing the receiver to drop them and remain in EXSTART. This occurs because OSPF uses the interface MTU to determine the maximum size of Database Description packets, and when an MTU mismatch exists, the larger DBD packet is silently discarded by the interface with the smaller MTU, preventing the adjacency from progressing past the EXSTART state. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding that OSPF does not negotiate MTU—it simply compares the MTU value in the DBD packet, and a mismatch stalls the process. A common trap is assuming OSPF will automatically fragment or adjust, but it will not; the adjacency hangs in EXSTART until the MTUs are matched. Memory tip: “Big DBDs get dropped by small MTUs—EXSTART is the dead-end for mismatched sizes.”

300-410 NetFlow and Flexible NetFlow Practice Question

This 300-410 practice question tests your understanding of netflow and flexible netflow. 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 OSPF on two directly connected routers with MTU 1500 on one interface and MTU 1400 on the other. The OSPF adjacency forms but remains in EXSTART state. Which 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.

Question 1hardmultiple choice

Review the full OSPF breakdown →

A
The router with the larger MTU sends DBD packets that exceed the smaller MTU, causing the receiver to drop them and remain in EXSTART.
OSPF DBD packets include the MTU of the sending interface. If the receiving interface has a smaller MTU, it will ignore the DBD packet, preventing the adjacency from progressing.
B
The router with the smaller MTU cannot send hello packets due to fragmentation, so the adjacency never forms.
Why wrong: Hello packets are small and typically not affected by MTU. The issue is with DBD packets, not hellos.
C
OSPF uses TCP, and the MTU mismatch causes TCP segmentation issues, leading to EXSTART.
Why wrong: OSPF uses IP protocol 89, not TCP. MTU affects IP fragmentation, not TCP segmentation.
D
The MTU mismatch causes a routing loop, preventing the exchange of LSAs.
Why wrong: MTU mismatch does not cause routing loops; it directly affects packet delivery at Layer 2/Layer 3.

Full breakdown with real-world context →

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 router with the larger MTU sends DBD packets that exceed the smaller MTU, causing the receiver to drop them and remain in EXSTART.

OSPF uses the MTU of the interface to determine the maximum size of Database Description (DBD) packets. If the MTU mismatch is such that the DBD packet from the larger MTU side is fragmented or dropped by the smaller MTU interface, the adjacency will stall in EXSTART. OSPF does not negotiate MTU; it simply compares the MTU value in the DBD packet. If the receiving interface has a smaller MTU, it will reject the DBD packet, causing the neighbor to stay in EXSTART.

Key principle: OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.

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 router with the larger MTU sends DBD packets that exceed the smaller MTU, causing the receiver to drop them and remain in EXSTART.
Why this is correct
OSPF DBD packets include the MTU of the sending interface. If the receiving interface has a smaller MTU, it will ignore the DBD packet, preventing the adjacency from progressing.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
OSPF neighbours must agree on key parameters.
✗
The router with the smaller MTU cannot send hello packets due to fragmentation, so the adjacency never forms.
Why it's wrong here
Hello packets are small and typically not affected by MTU. The issue is with DBD packets, not hellos.
✗
OSPF uses TCP, and the MTU mismatch causes TCP segmentation issues, leading to EXSTART.
Why it's wrong here
OSPF uses IP protocol 89, not TCP. MTU affects IP fragmentation, not TCP segmentation.
✗
The MTU mismatch causes a routing loop, preventing the exchange of LSAs.
Why it's wrong here
MTU mismatch does not cause routing loops; it directly affects packet delivery at Layer 2/Layer 3.

Common exam traps

Common exam trap: OSPF can fail even when IP connectivity looks correct

OSPF neighbour formation depends on matching areas, timers, network type, authentication and passive-interface behaviour. Do not choose an answer only because the devices can ping.

Detailed technical explanation

How to think about this question

OSPF questions usually test the details that control adjacency and route selection. Read the neighbour state, area, router ID and interface configuration before deciding what is wrong.

KKey Concepts to Remember

OSPF neighbours must agree on key parameters.
Router ID selection can affect neighbour relationships and LSDB output.
OSPF cost influences the preferred path.
A route can appear in OSPF information but not become the installed route.

TExam Day Tips

Check area mismatch first when OSPF adjacency fails.
Review passive interfaces when a network is advertised but no neighbour forms.
Use show ip ospf neighbor and show ip route clues carefully.

Key takeaway

OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.

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.

Review OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related 300-410 OSPF questions on adjacency and route selection.

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FAQ

Questions learners often ask

What does this 300-410 question test?

NetFlow and Flexible NetFlow — This question tests NetFlow and Flexible NetFlow — OSPF neighbours must agree on key parameters..

What is the correct answer to this question?

The correct answer is: The router with the larger MTU sends DBD packets that exceed the smaller MTU, causing the receiver to drop them and remain in EXSTART. — OSPF uses the MTU of the interface to determine the maximum size of Database Description (DBD) packets. If the MTU mismatch is such that the DBD packet from the larger MTU side is fragmented or dropped by the smaller MTU interface, the adjacency will stall in EXSTART. OSPF does not negotiate MTU; it simply compares the MTU value in the DBD packet. If the receiving interface has a smaller MTU, it will reject the DBD packet, causing the neighbor to stay in EXSTART.

What should I do if I get this 300-410 question wrong?

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?

OSPF neighbours must agree on key parameters.

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