- A
The OSPF network type is point-to-point on one router and broadcast on the other.
Why wrong: Incorrect because OSPF network type mismatch typically causes issues in the 2WAY or FULL state, not EXSTART. The EXSTART state is specifically for DBD exchange.
- B
The OSPF hello and dead intervals are mismatched.
Why wrong: Incorrect because mismatched timers prevent the adjacency from forming at all, typically leaving the routers in the INIT state.
- C
One router has a lower IP MTU configured on the interface, causing the DBD packet to be dropped.
Correct because OSPF routers exchange DBD packets in the EXSTART state. If the DBD packet size exceeds the IP MTU, the packet is dropped, and the routers remain stuck in EXSTART.
- D
The OSPF router IDs are the same.
Why wrong: Incorrect because duplicate router IDs prevent adjacency formation entirely, typically resulting in a state of DOWN or INIT, not EXSTART.
Quick Answer
The answer is an IP MTU mismatch, specifically where one router has a lower IP MTU configured on the interface, causing the Database Description (DBD) packet to be dropped. Even when both routers show a configured MTU of 1500 bytes, factors like encapsulation overhead or a different interface MTU can reduce the actual IP MTU on one side. Since OSPF DBD packets are never fragmented, the larger packet is silently discarded, leaving the routers stuck in the EXSTART state. On the ENCOR 350-401 exam, this question tests your understanding that OSPF adjacency progression depends on the *effective* IP MTU, not just the configured value—a common trap is assuming identical configured MTUs guarantee compatibility. Remember the memory tip: “EXSTART equals DBD exchange, and DBDs don’t fragment—so if they’re stuck, check the real IP MTU, not the configured one.”
CCNP OSPF Practice Question
This 350-401 practice question tests your understanding of ospf. 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 OSPF adjacency issues between two routers connected via a Gigabit Ethernet link. The engineer notices that the routers are stuck in the EXSTART state. Both routers have the same MTU of 1500 bytes. What is the most likely cause of this issue?
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
One router has a lower IP MTU configured on the interface, causing the DBD packet to be dropped.
When OSPF routers are stuck in the EXSTART state, it typically indicates a problem with the Database Description (DBD) packet exchange. Even though both routers have the same configured MTU of 1500 bytes, one router may have a lower IP MTU on its interface (e.g., due to a different interface MTU or encapsulation overhead), causing the DBD packet to be fragmented or dropped. Since OSPF DBD packets are not fragmented, a mismatch in the actual IP MTU prevents the adjacency from progressing beyond EXSTART.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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 OSPF network type is point-to-point on one router and broadcast on the other.
Why it's wrong here
Incorrect because OSPF network type mismatch typically causes issues in the 2WAY or FULL state, not EXSTART. The EXSTART state is specifically for DBD exchange.
- ✗
The OSPF hello and dead intervals are mismatched.
Why it's wrong here
Incorrect because mismatched timers prevent the adjacency from forming at all, typically leaving the routers in the INIT state.
- ✓
One router has a lower IP MTU configured on the interface, causing the DBD packet to be dropped.
Why this is correct
Correct because OSPF routers exchange DBD packets in the EXSTART state. If the DBD packet size exceeds the IP MTU, the packet is dropped, and the routers remain stuck in EXSTART.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
The OSPF router IDs are the same.
Why it's wrong here
Incorrect because duplicate router IDs prevent adjacency formation entirely, typically resulting in a state of DOWN or INIT, not EXSTART.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the nuance that the configured MTU (e.g., 1500 bytes) may not equal the actual IP MTU due to overhead from encapsulation or interface settings, leading to DBD packet drops and a stuck EXSTART state.
Detailed technical explanation
How to think about this question
OSPF DBD packets are sent using IP protocol 89 and are not fragmented; if the actual IP MTU (which may be lower than the configured MTU due to features like PPPoE, GRE tunnels, or VLAN tagging) is less than the size of the DBD packet, the packet is silently dropped. This is a common issue on Ethernet links where the interface MTU is set to 1500 but the IP MTU is effectively lower due to additional headers. The 'ip mtu' command on Cisco IOS can be used to explicitly set the IP MTU, and a mismatch as small as 4 bytes can cause the adjacency to stall in EXSTART.
KKey Concepts to Remember
- Read the scenario before looking for a memorised answer.
- Find the constraint that changes the correct option.
- Eliminate answers that are true in general but not in this case.
TExam Day Tips
- Watch for words such as best, first, most likely and least administrative effort.
- Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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FAQ
Questions learners often ask
What does this 350-401 question test?
OSPF — This question tests OSPF — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: One router has a lower IP MTU configured on the interface, causing the DBD packet to be dropped. — When OSPF routers are stuck in the EXSTART state, it typically indicates a problem with the Database Description (DBD) packet exchange. Even though both routers have the same configured MTU of 1500 bytes, one router may have a lower IP MTU on its interface (e.g., due to a different interface MTU or encapsulation overhead), causing the DBD packet to be fragmented or dropped. Since OSPF DBD packets are not fragmented, a mismatch in the actual IP MTU prevents the adjacency from progressing beyond EXSTART.
What should I do if I get this 350-401 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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?
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 24, 2026
This 350-401 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 350-401 exam.
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