The correct answer is an MTU mismatch between the interfaces. This is because during the EXSTART state, OSPF neighbors negotiate the master/slave relationship and exchange Database Description (DBD) packets; if one interface has a larger MTU than the other, the oversized DBD packet is silently dropped, preventing the adjacency from progressing past EXSTART. On the Cisco DCCOR 350-601 exam, this scenario often appears in troubleshooting exhibits where a neighbor is stuck in EXSTART or EXCHANGE, and the trap is to overlook MTU while focusing on authentication or area mismatches. A quick memory tip: “EXSTART and EXCHANGE—if DBDs don’t fit, the adjacency will quit.”
350-601 Network Practice Question
This 350-601 practice question tests your understanding of network. 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.
Exhibit
show ip ospf neighbor
Neighbor ID Pri State Dead Time Address Interface
10.1.1.1 1 FULL/DR 00:00:38 192.168.1.2 Ethernet1/1
10.1.1.2 1 2WAY/DROTHER 00:00:34 192.168.1.3 Ethernet1/1
10.1.1.3 1 EXSTART/DR 00:00:32 192.168.1.4 Ethernet1/1
Refer to the exhibit. What is the most likely cause of neighbor 10.1.1.3 being stuck in EXSTART?
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.
show ip ospf neighbor
Neighbor ID Pri State Dead Time Address Interface
10.1.1.1 1 FULL/DR 00:00:38 192.168.1.2 Ethernet1/1
10.1.1.2 1 2WAY/DROTHER 00:00:34 192.168.1.3 Ethernet1/1
10.1.1.3 1 EXSTART/DR 00:00:32 192.168.1.4 Ethernet1/1
A
Duplicate router ID.
Why wrong: Duplicate RID would cause a neighbor to go down or be rejected.
B
OSPF network type mismatch.
Why wrong: Network type mismatch prevents DR election, but not typically EXSTART.
C
MTU mismatch between the interfaces.
MTU mismatch prevents DBD packets from being sent successfully.
D
The interface is configured as passive.
Why wrong: Passive interfaces do not form adjacencies.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
MTU mismatch between the interfaces.
In OSPF, the EXSTART state indicates that neighbors are negotiating the master/slave relationship and exchanging Database Description (DBD) packets. If the MTU of the interface on one side is larger than the MTU on the other, the larger DBD packet will be silently dropped, preventing the neighbor from progressing past EXSTART. This is a classic symptom of an MTU mismatch, as the OSPF adjacency will remain stuck in EXSTART or EXCHANGE.
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.
✗
Duplicate router ID.
Why it's wrong here
Duplicate RID would cause a neighbor to go down or be rejected.
✗
OSPF network type mismatch.
Why it's wrong here
Network type mismatch prevents DR election, but not typically EXSTART.
✓
MTU mismatch between the interfaces.
Why this is correct
MTU mismatch prevents DBD packets from being sent successfully.
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 interface is configured as passive.
Why it's wrong here
Passive interfaces do not form adjacencies.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the MTU mismatch trap by having candidates confuse it with a network type mismatch, but the key differentiator is that MTU issues cause the adjacency to stall specifically in EXSTART/EXCHANGE, while network type mismatches prevent the adjacency from forming past INIT/2WAY.
Detailed technical explanation
How to think about this question
OSPF uses the MTU of the interface to determine the maximum size of DBD packets; if the received DBD packet exceeds the interface MTU, it is dropped without any error message, causing the neighbor to retransmit indefinitely. This behavior is defined in RFC 2328, Section 10.6, and can be verified by checking the 'show ip ospf neighbor' output for a stuck EXSTART state and comparing interface MTUs with 'show interfaces'. In real-world scenarios, this often occurs when one interface has an MTU of 1500 and the other has 1492 (e.g., due to PPPoE overhead), and the fix is to manually set the MTU to match on both sides.
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.
Network — This question tests Network — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: MTU mismatch between the interfaces. — In OSPF, the EXSTART state indicates that neighbors are negotiating the master/slave relationship and exchanging Database Description (DBD) packets. If the MTU of the interface on one side is larger than the MTU on the other, the larger DBD packet will be silently dropped, preventing the neighbor from progressing past EXSTART. This is a classic symptom of an MTU mismatch, as the OSPF adjacency will remain stuck in EXSTART or EXCHANGE.
What should I do if I get this 350-601 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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