- A
The MTU on one of the interfaces is lower than the other, causing DBD packet fragmentation.
Correct. A lower MTU on one side causes DBD packets to be dropped, preventing the adjacency from leaving EXSTART.
- B
The OSPF hello interval is mismatched between the two routers.
Why wrong: Incorrect. A hello interval mismatch would prevent the adjacency from forming at all, not cause flapping between FULL and EXSTART.
- C
The OSPF dead interval is too short, causing the neighbor to be declared dead prematurely.
Why wrong: Incorrect. A short dead interval would cause the neighbor to go DOWN, not to EXSTART.
- D
The OSPF router ID is duplicated on the segment.
Why wrong: Incorrect. Duplicate router IDs would cause one router to reject the other, but the adjacency would not reach FULL and then flap to EXSTART.
Quick Answer
The answer is an MTU mismatch on the directly connected GigabitEthernet interfaces. This is correct because OSPF uses the interface MTU to determine the maximum size of Database Description (DBD) packets during the EXSTART state; if one router sends a DBD packet larger than the receiving interface’s MTU, the packet is silently dropped, causing the adjacency to repeatedly flap between FULL and EXSTART rather than completing the exchange. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding of OSPF neighbor state progression and the often-overlooked impact of MTU on DBD packet fragmentation, especially in MPLS L3VPN backbones where overhead from labels can reduce the effective MTU. A common trap is to focus on mismatched OSPF network types or authentication, but the key clue is the flapping between FULL and EXSTART—not a permanent stuck state. Memory tip: “MTU mismatch makes DBDs ditch, so EXSTART flips and flops.”
300-410 MPLS L3VPN Practice Question
This 300-410 practice question tests your understanding of mpls l3vpn. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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 between two PE routers in an MPLS L3VPN backbone. Both routers are directly connected via a GigabitEthernet link. The OSPF adjacency forms and then repeatedly flaps between FULL and EXSTART. The engineer verifies that both routers have matching OSPF network types (broadcast), matching area IDs, and no authentication mismatch. What is the most likely cause of the flapping?
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 MTU on one of the interfaces is lower than the other, causing DBD packet fragmentation.
In OSPF, if the MTU of the interface on one side is smaller than the size of a Database Description (DBD) packet, the neighbor will be stuck in EXSTART or the adjacency will flap. This is because OSPF uses the interface MTU to determine the maximum size of DBD packets, and if a router receives a DBD packet larger than its interface MTU, it drops it and the neighbor relationship cannot progress. This is a common edge case when one interface has a reduced MTU (e.g., due to MPLS overhead or misconfiguration) while the other uses the default 1500 bytes.
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 MTU on one of the interfaces is lower than the other, causing DBD packet fragmentation.
- ✗
The OSPF hello interval is mismatched between the two routers.
Why it's wrong here
Incorrect. A hello interval mismatch would prevent the adjacency from forming at all, not cause flapping between FULL and EXSTART.
- ✗
The OSPF dead interval is too short, causing the neighbor to be declared dead prematurely.
Why it's wrong here
Incorrect. A short dead interval would cause the neighbor to go DOWN, not to EXSTART.
- ✗
The OSPF router ID is duplicated on the segment.
Why it's wrong here
Incorrect. Duplicate router IDs would cause one router to reject the other, but the adjacency would not reach FULL and then flap to 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.
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?
MPLS L3VPN — This question tests MPLS L3VPN — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: The MTU on one of the interfaces is lower than the other, causing DBD packet fragmentation. — In OSPF, if the MTU of the interface on one side is smaller than the size of a Database Description (DBD) packet, the neighbor will be stuck in EXSTART or the adjacency will flap. This is because OSPF uses the interface MTU to determine the maximum size of DBD packets, and if a router receives a DBD packet larger than its interface MTU, it drops it and the neighbor relationship cannot progress. This is a common edge case when one interface has a reduced MTU (e.g., due to MPLS overhead or misconfiguration) while the other uses the default 1500 bytes.
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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