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 OSPFv2 adjacency issue between two routers connected via a VLAN trunk. R1 and R2 are in different VLANs but are connected through a Layer 2 switch. The engineer has configured 'ip ospf 1 area 0' on the subinterfaces. The adjacency forms but is stuck in INIT state. 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 routers have the same OSPF router ID.
The INIT state indicates that the router has received a hello packet from the neighbor but the neighbor has not seen its own router ID in the hello packet. This is often due to a mismatch in the OSPF network type or a unicast communication issue. In a VLANtrunk scenario, if the subinterfaces are not in the same VLAN, the routers may not be able to communicate at Layer 2. However, the most common cause is that the routers are using the same router ID, which causes the hello packet to be ignored because the router sees its own router ID in the neighbor list.
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 routers have the same OSPF router ID.
Why this is correct
When a router receives a hello packet with its own router ID, it ignores it, causing the adjacency to stay in INIT.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
The OSPF hello interval is mismatched between the two routers.
Why it's wrong here
A hello interval mismatch would cause the adjacency to not form at all, not to be stuck in INIT.
✗
The subinterfaces are in different VLANs, preventing Layer 2 communication.
Why it's wrong here
If they were in different VLANs, the routers would not receive each other's hellos, so no adjacency would form.
✗
The OSPF network type is set to point-to-point on one side and broadcast on the other.
Why it's wrong here
A network type mismatch would cause the adjacency to be stuck in EXSTART or EXCHANGE, not INIT.
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 routers have the same OSPF router ID. — The INIT state indicates that the router has received a hello packet from the neighbor but the neighbor has not seen its own router ID in the hello packet. This is often due to a mismatch in the OSPF network type or a unicast communication issue. In a VLAN trunk scenario, if the subinterfaces are not in the same VLAN, the routers may not be able to communicate at Layer 2. However, the most common cause is that the routers are using the same router ID, which causes the hello packet to be ignored because the router sees its own router ID in the neighbor list.
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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