The answer is an MTU mismatch between the two routers, specifically where one interface has an MTU of 1500 and the other 1400. This is correct because OSPF neighbors become stuck in the EXSTART state when Database Description (DBD) packets are dropped due to a size mismatch; the DBD packet size is derived from the interface MTU, so if the receiving interface has a smaller MTU, the packet is silently discarded, preventing the adjacency from progressing past the exchange of master/slave roles. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding of OSPF neighbor state machine mechanics and Layer 2 constraints—a common trap is to blame a mismatched OSPF network type or authentication, but the key clue is that the routers reach EXSTART but never exchange DBDs. Remember the memory tip: "MTU mismatch kills the DBD handshake, leaving neighbors stuck in EXSTART."
300-410 OSPF Troubleshooting (v2/v3) Practice Question
This 300-410 practice question tests your understanding of ospf troubleshooting (v2/v3). 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 OSPFv2 on two routers with a direct Ethernet link. The routers are stuck in the EXSTART 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 the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The interface MTU on one router is 1500, and on the other is 1400.
OSPF neighbors can become stuck in EXSTART when there is a mismatch in the interface MTU. The DBD packet size is based on the interface MTU; if the receiving interface has a smaller MTU, the DBD packet is dropped, preventing the adjacency from progressing.
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 interface MTU on one router is 1500, and on the other is 1400.
Why this is correct
An MTU mismatch causes DBD packets to be dropped, stalling the adjacency in EXSTART.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Mismatched hello/dead intervals prevent the adjacency from forming at all, not getting stuck in EXSTART.
✗
One router has OSPF authentication configured, and the other does not.
Why it's wrong here
Authentication mismatch prevents neighbor state progression past INIT or 2-WAY, not EXSTART.
✗
The OSPF network type is point-to-point on one router and broadcast on the other.
Why it's wrong here
Network type mismatch typically prevents adjacency or causes it to stall at INIT, 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 interface MTU on one router is 1500, and on the other is 1400. — OSPF neighbors can become stuck in EXSTART when there is a mismatch in the interface MTU. The DBD packet size is based on the interface MTU; if the receiving interface has a smaller MTU, the DBD packet is dropped, preventing the adjacency from progressing.
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
Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →
Share a tip, memory trick, or ask about the reasoning behind this question. Do not post real exam questions, leaked content, braindumps, or copyrighted exam material. Comments are moderated and may be removed without notice.
This 300-410 practice question is part of Courseiva's free Cisco certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 300-410 exam.
Question Discussion
Share a tip, memory trick, or ask about the reasoning behind this question. Do not post real exam questions, leaked content, braindumps, or copyrighted exam material. Comments are moderated and may be removed without notice.
Sign in to join the discussion.