The answer is the router with the smaller MTU rejects DBD packets that exceed its MTU, preventing the exchange of LSAs. This occurs because OSPF uses the interface MTU to size Database Description packets; when a mismatch exists, the neighbor with the lower MTU silently drops any DBD packet larger than its configured limit, halting the adjacency in the EXSTART state. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding of OSPF neighbor state progression and the specific role of MTU in the DBD exchange process—a common trap is assuming the adjacency will fail to form at all, when in fact it gets stuck in EXSTART. A reliable memory tip is “smaller MTU, stuck in EXSTART”: the router with the lower MTU is the one that blocks the DBD packets, not the larger one.
300-410 Administrative Distance Practice Question
This 300-410 practice question tests your understanding of administrative distance. 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 routers connected via a serial link. The MTU on one side is 1500 and on the other is 1400. The OSPF adjacency forms but stays stuck 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.
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 smaller MTU rejects DBD packets that exceed its MTU, preventing the exchange of LSAs.
OSPF uses the interface MTU to determine the maximum size of Database Description (DBD) packets. If the MTU mismatches, the router with the smaller MTU will reject DBD packets from the neighbor, causing the adjacency to remain in EXSTART state.
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 cannot process OSPF hello packets from the neighbor.
Why it's wrong here
Hello packets are small and not affected by MTU mismatch.
✓
The router with the smaller MTU rejects DBD packets that exceed its MTU, preventing the exchange of LSAs.
Why this is correct
DBD packets must fit within the interface MTU; a mismatch causes the adjacency to stall in EXSTART.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
The OSPF network type must be point-to-point for serial links; otherwise, the adjacency fails.
Why it's wrong here
Network type affects DR/BDR election but not MTU handling.
✗
The OSPF process ID must match on both routers for adjacency to form.
Why it's wrong here
Process ID is locally significant and does not need to match.
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.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 300-410 question in full detail.
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.
Administrative Distance — This question tests Administrative Distance — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: The router with the smaller MTU rejects DBD packets that exceed its MTU, preventing the exchange of LSAs. — OSPF uses the interface MTU to determine the maximum size of Database Description (DBD) packets. If the MTU mismatches, the router with the smaller MTU will reject DBD packets from the neighbor, causing the adjacency to remain in EXSTART state.
What should I do if I get this 300-410 question wrong?
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.
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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