This 300-410 practice question tests your understanding of route summarization. Examine the command output carefully: the correct answer depends on what the output actually shows, not on general recall alone. 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 point-to-point link. The routers are stuck in EXSTART state. 'show ip ospf neighbor' shows neighbor state EXSTART/EXCHANGE. 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 MTU on the interfaces is mismatched.
In OSPF, the EXSTART state involves negotiating the master/slave relationship and the Database Description (DBD) packet size. If the MTU on the interfaces differs, the larger DBD packet from the router with the larger MTU will be dropped by the router with the smaller MTU, causing the adjacency to remain stuck in EXSTART. This is a common edge case that is often overlooked.
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 OSPF network type is mismatched (point-to-point vs broadcast).
Why it's wrong here
A network type mismatch typically causes the neighbor to get stuck in INIT or 2-WAY, not EXSTART.
✓
The MTU on the interfaces is mismatched.
Why this is correct
An MTU mismatch prevents DBD packets from being exchanged, causing the adjacency to remain in EXSTART.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Duplicate router IDs would prevent the adjacency from forming at all, typically resulting in a state of DOWN or INIT.
✗
The hello/dead intervals are mismatched.
Why it's wrong here
Mismatched hello/dead intervals would prevent the neighbor from reaching the 2-WAY state, not EXSTART.
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.
Route Summarization — This question tests Route Summarization — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: The MTU on the interfaces is mismatched. — In OSPF, the EXSTART state involves negotiating the master/slave relationship and the Database Description (DBD) packet size. If the MTU on the interfaces differs, the larger DBD packet from the router with the larger MTU will be dropped by the router with the smaller MTU, causing the adjacency to remain stuck in EXSTART. This is a common edge case that is often overlooked.
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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