- A
The larger MTU interface sends DBD packets that are too big for the smaller MTU interface, preventing the exchange of LSAs.
OSPF DBD packets are limited by the outgoing interface MTU; a mismatch causes the smaller MTU router to drop the packets, stalling the adjacency.
- B
OSPF uses TCP, and the MSS mismatch causes the adjacency to fail.
Why wrong: OSPF uses IP protocol 89, not TCP, so MSS is irrelevant.
- C
The MTU mismatch causes a routing loop that prevents the exchange of hello packets.
Why wrong: MTU mismatch does not affect hello packets; they are small and always fit within any MTU.
- D
The router with the smaller MTU will not send hello packets because it detects the mismatch.
Why wrong: Hello packets are not affected by MTU mismatch; the issue is with DBD packets.
Quick Answer
The answer is that the larger MTU interface sends DBD packets that are too big for the smaller MTU interface, preventing the exchange of LSAs. This occurs because OSPF embeds the interface MTU into Database Description packets, and a router receiving a DBD packet larger than its own configured MTU will silently drop it, stalling the adjacency in EXSTART/EXCHANGE. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding that OSPF does not negotiate MTU—it simply enforces it, making MTU mismatch a classic trap when both sides are not explicitly set to the same value. Many engineers mistakenly assume OSPF will fragment or adapt, but the protocol requires identical MTU for DBD exchange to proceed. A reliable memory tip: "MTU must match, or EXSTART will catch."
300-410 Device Management Practice Question
This 300-410 practice question tests your understanding of device management. 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 1600 on the other. The OSPF adjacency remains stuck in EXSTART/EXCHANGE 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 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 larger MTU interface sends DBD packets that are too big for the smaller MTU interface, preventing the exchange of LSAs.
OSPF uses the interface MTU in the Database Description (DBD) packets. If the MTU values differ, the receiving router will reject DBD packets larger than its own MTU, causing the adjacency to stall in EXSTART/EXCHANGE. This is a common edge case because many engineers assume OSPF will negotiate MTU automatically.
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 larger MTU interface sends DBD packets that are too big for the smaller MTU interface, preventing the exchange of LSAs.
Why this is correct
OSPF DBD packets are limited by the outgoing interface MTU; a mismatch causes the smaller MTU router to drop the packets, stalling the adjacency.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
OSPF neighbours must agree on key parameters.
- ✗
OSPF uses TCP, and the MSS mismatch causes the adjacency to fail.
- ✗
The MTU mismatch causes a routing loop that prevents the exchange of hello packets.
Why it's wrong here
MTU mismatch does not affect hello packets; they are small and always fit within any MTU.
- ✗
The router with the smaller MTU will not send hello packets because it detects the mismatch.
Why it's wrong here
Hello packets are not affected by MTU mismatch; the issue is with DBD packets.
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?
Device Management — This question tests Device Management — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: The larger MTU interface sends DBD packets that are too big for the smaller MTU interface, preventing the exchange of LSAs. — OSPF uses the interface MTU in the Database Description (DBD) packets. If the MTU values differ, the receiving router will reject DBD packets larger than its own MTU, causing the adjacency to stall in EXSTART/EXCHANGE. This is a common edge case because many engineers assume OSPF will negotiate MTU automatically.
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.
About these practice questions
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Same concept, more angles
1 more ways this is tested on 300-410
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. A network engineer runs the following command to troubleshoot an OSPF adjacency issue: R1# debug ip ospf adj OSPF: Interface GigabitEthernet0/0 going Up OSPF: Send with youngest orig age 0 OSPF: Rcv DBD from 2.2.2.2 seq 0x1A opt 0x52 flag 0x7 len 32 mtu 1500 state INIT OSPF: First DBD and we are not SLAVE OSPF: Rcv DBD from 2.2.2.2 seq 0x1A opt 0x52 flag 0x7 len 32 mtu 1500 state EXSTART OSPF: Nbr 2.2.2.2 has larger interface MTU What does this output indicate?
medium- A.OSPF adjacency is forming correctly, moving to FULL state.
- ✓ B.OSPF adjacency is stuck due to MTU mismatch; the neighbor has a larger MTU.
- C.OSPF adjacency is stuck because the router is not the DR/BDR.
- D.OSPF adjacency is stuck due to authentication mismatch.
Why B: The debug shows that the OSPF neighbor has a larger interface MTU, which prevents the adjacency from forming. OSPF requires matching MTU values on the link.
Last reviewed: Jun 18, 2026
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