- A
The router with the smaller MTU (1400) rejects DBD packets from the neighbor with MTU 1500 because they exceed the interface MTU, causing the EXSTART state.
OSPF DBD packets are sized based on MTU; a mismatch prevents the exchange from completing.
- B
The router with the larger MTU (1500) cannot process the smaller DBD packets from the neighbor.
Why wrong: Smaller packets are not a problem; the issue is larger packets being fragmented or dropped.
- C
The OSPF network type is point-to-point, which ignores MTU mismatches.
Why wrong: Point-to-point networks still use DBD packets and are affected by MTU mismatch.
- D
The 'ip ospf mtu-ignore' command is needed on both sides to bypass the MTU check.
Why wrong: This command can fix the issue, but the question asks for the explanation of the problem.
300-410 BGP Troubleshooting Practice Question
This 300-410 practice question tests your understanding of bgp troubleshooting. 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 a link with an MTU of 1500 on one side and 1400 on the other. The OSPF neighbor state is stuck 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 router with the smaller MTU (1400) rejects DBD packets from the neighbor with MTU 1500 because they exceed the interface MTU, causing the EXSTART state.
OSPF uses the MTU of the interface to determine the maximum size of Database Description (DBD) packets. If the MTU mismatches, the router with the smaller MTU will reject DBD packets that exceed its MTU, causing the neighbor to remain 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 smaller MTU (1400) rejects DBD packets from the neighbor with MTU 1500 because they exceed the interface MTU, causing the EXSTART state.
- ✗
The router with the larger MTU (1500) cannot process the smaller DBD packets from the neighbor.
Why it's wrong here
Smaller packets are not a problem; the issue is larger packets being fragmented or dropped.
- ✗
The OSPF network type is point-to-point, which ignores MTU mismatches.
Why it's wrong here
Point-to-point networks still use DBD packets and are affected by MTU mismatch.
- ✗
The 'ip ospf mtu-ignore' command is needed on both sides to bypass the MTU check.
Why it's wrong here
This command can fix the issue, but the question asks for the explanation of the problem.
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.
Trap categories for this question
Command / output trap
This command can fix the issue, but the question asks for the explanation of the problem.
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?
BGP Troubleshooting — This question tests BGP Troubleshooting — 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 (1400) rejects DBD packets from the neighbor with MTU 1500 because they exceed the interface MTU, causing the EXSTART state. — OSPF uses the MTU of the interface to determine the maximum size of Database Description (DBD) packets. If the MTU mismatches, the router with the smaller MTU will reject DBD packets that exceed its MTU, causing the neighbor to remain in EXSTART.
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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Last reviewed: Jun 18, 2026
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