300-410 OSPF Troubleshooting (v2/v3) Practice Question
This 300-410 practice question tests your understanding of ospf troubleshooting (v2/v3). 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.
A network engineer is troubleshooting an OSPFv3 adjacency issue between two routers over an IPv6-only link. The engineer configures OSPFv3 on both routers and sees that the adjacency is stuck in EXSTART state. 'debug ipv6 ospf packet' shows that the routers are exchanging DBD packets but not progressing. What is the most likely cause?
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 OSPFv3 interface MTU is mismatched between the two routers.
The EXSTART state indicates that the routers are trying to establish the master/slave relationship and exchange DBD packets. If they are stuck, it is often due to a mismatch in the OSPFMTU or the interface MTU. If the MTU is mismatched, the DBD packets may be too large to be sent, causing the routers to keep retransmitting. Another cause could be a mismatch in the OSPF network type, but that usually causes issues earlier. The most likely cause is an MTU mismatch.
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 OSPFv3 interface MTU is mismatched between the two routers.
Why this is correct
An MTU mismatch prevents DBD packets from being exchanged properly, causing the adjacency to be stuck in EXSTART.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Different router IDs are required for adjacency, so this would not cause an issue.
✗
The OSPFv3 process is configured with 'ipv6 ospf 1 area 0' on the interface, but the interface is not IPv6-enabled.
Why it's wrong here
If the interface were not IPv6-enabled, OSPFv3 would not even start the adjacency process.
✗
The link is a point-to-point link, but the OSPF network type is set to broadcast.
Why it's wrong here
A network type mismatch would cause the adjacency to be stuck in INIT or 2WAY, 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.
OSPF Troubleshooting (v2/v3) — This question tests OSPF Troubleshooting (v2/v3) — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: The OSPFv3 interface MTU is mismatched between the two routers. — The EXSTART state indicates that the routers are trying to establish the master/slave relationship and exchange DBD packets. If they are stuck, it is often due to a mismatch in the OSPF MTU or the interface MTU. If the MTU is mismatched, the DBD packets may be too large to be sent, causing the routers to keep retransmitting. Another cause could be a mismatch in the OSPF network type, but that usually causes issues earlier. The most likely cause is an MTU mismatch.
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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