This 200-301 practice question tests your understanding of ip routing. 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
R1# show ip ospf neighbor
Neighbor ID Pri State Dead Time Address Interface
10.0.0.2 1 EXSTART/DROTHER 00:00:35 10.0.0.2 GigabitEthernet0/0
10.0.1.2 1 FULL/DROTHER 00:00:38 10.0.1.2 GigabitEthernet0/1
192.168.1.1 1 FULL/DR 00:00:37 192.168.1.1 GigabitEthernet0/2
Refer to the exhibit. A network engineer is troubleshooting an OSPF adjacency issue between R1 and R2. The output of the show ip ospf neighbor command on R1 shows the neighbor relationship with R2 stuck in the EXSTART/DROTHER 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.
R1# show ip ospf neighbor
Neighbor ID Pri State Dead Time Address Interface
10.0.0.2 1 EXSTART/DROTHER 00:00:35 10.0.0.2 GigabitEthernet0/0
10.0.1.2 1 FULL/DROTHER 00:00:38 10.0.1.2 GigabitEthernet0/1
192.168.1.1 1 FULL/DR 00:00:37 192.168.1.1 GigabitEthernet0/2
A
OSPF network type mismatch between R1 and R2, preventing full adjacency.
Why wrong: A network type mismatch would cause the neighbor state to become 2-WAY/DROTHER (if both ends are on a broadcast network but parameters differ) rather than EXSTART/DROTHER. The output shows EXSTART, pointing to a DBD exchange problem.
B
Duplicate OSPF Router IDs on R1 and R2, causing DBD packet rejection.
Why wrong: If the Router IDs were identical, OSPF would detect a duplicate and the neighbor would not appear in the neighbor table; the hello would be ignored. The presence of the neighbor in EXSTART proves the Router IDs are different.
C
OSPF authentication mismatch on the link, causing DBD packets to be rejected.
Why wrong: Authentication failures occur during Hello packet processing, not DBD exchange. If authentication were mismatched, the neighbor would not appear in the output because Hellos would be dropped before forming a neighbor relationship.
D
MTU mismatch on the link between R1 and R2, causing DBD packets to be dropped.
The EXSTART/DROTHER state in the exhibit indicates that OSPF is stuck in the DBD exchange phase. This is a classic symptom of an MTU mismatch, where one side creates DBD packets larger than the other's MTU, leading to silent drops. The output directly confirms the neighbor is in EXSTART, not EXCHANGE or FULL.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
MTU mismatch on the link between R1 and R2, causing DBD packets to be dropped.
The output specifically shows neighbor 10.0.0.2 in the EXSTART/DROTHER state. This state indicates that the Database Description (DBD) packet exchange phase has not completed. The most common cause for a neighbor to remain stuck in EXSTART is an MTU mismatch on the link, causing DBD packets (which are larger than the MTU) to be silently dropped.
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.
✗
OSPF network type mismatch between R1 and R2, preventing full adjacency.
Why it's wrong here
A network type mismatch would cause the neighbor state to become 2-WAY/DROTHER (if both ends are on a broadcast network but parameters differ) rather than EXSTART/DROTHER. The output shows EXSTART, pointing to a DBD exchange problem.
✗
Duplicate OSPF Router IDs on R1 and R2, causing DBD packet rejection.
Why it's wrong here
If the Router IDs were identical, OSPF would detect a duplicate and the neighbor would not appear in the neighbor table; the hello would be ignored. The presence of the neighbor in EXSTART proves the Router IDs are different.
✗
OSPF authentication mismatch on the link, causing DBD packets to be rejected.
Why it's wrong here
Authentication failures occur during Hello packet processing, not DBD exchange. If authentication were mismatched, the neighbor would not appear in the output because Hellos would be dropped before forming a neighbor relationship.
✓
MTU mismatch on the link between R1 and R2, causing DBD packets to be dropped.
Why this is correct
The EXSTART/DROTHER state in the exhibit indicates that OSPF is stuck in the DBD exchange phase. This is a classic symptom of an MTU mismatch, where one side creates DBD packets larger than the other's MTU, leading to silent drops. The output directly confirms the neighbor is in EXSTART, not EXCHANGE or FULL.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Understanding why wrong answers are wrong — and when they would be correct — is what separates a 750 score from a 900. The 200-301 exam frequently reuses these exact scenarios with slightly different constraints.
✓MTU mismatch on the link between R1 and R2, causing DBD packets to be dropped.Correct answer▾
Why this is correct
The EXSTART/DROTHER state in the exhibit indicates that OSPF is stuck in the DBD exchange phase. This is a classic symptom of an MTU mismatch, where one side creates DBD packets larger than the other's MTU, leading to silent drops. The output directly confirms the neighbor is in EXSTART, not EXCHANGE or FULL.
✗OSPF network type mismatch between R1 and R2, preventing full adjacency.Wrong answer — click to see why▾
Why this is wrong here
Candidates may confuse adjacency failures with DBD exchange problems, but network type mismatch leads to a different state (2-WAY).
✗Duplicate OSPF Router IDs on R1 and R2, causing DBD packet rejection.Wrong answer — click to see why▾
Why this is wrong here
Candidates think duplicate Router IDs might cause DBD exchange failure, but duplicate IDs prevent neighbor discovery entirely.
✗OSPF authentication mismatch on the link, causing DBD packets to be rejected.Wrong answer — click to see why▾
Why this is wrong here
Many associate EXSTART with any adjacency issue, but authentication errors prevent the neighbor from being listed at all.
Analysis generated from the official 200-301blueprint and verified against question context. The “when correct” sections are what AI assistants cite when candidates ask “what’s the difference between these options?”
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.
Trap categories for this question
Command / output trap
A network type mismatch would cause the neighbor state to become 2-WAY/DROTHER (if both ends are on a broadcast network but parameters differ) rather than EXSTART/DROTHER. The output shows EXSTART, pointing to a DBD exchange problem.
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 200-301 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 200-301 OSPF questions on adjacency and route selection.
IP Routing — This question tests IP Routing — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: MTU mismatch on the link between R1 and R2, causing DBD packets to be dropped. — The output specifically shows neighbor 10.0.0.2 in the EXSTART/DROTHER state. This state indicates that the Database Description (DBD) packet exchange phase has not completed. The most common cause for a neighbor to remain stuck in EXSTART is an MTU mismatch on the link, causing DBD packets (which are larger than the MTU) to be silently dropped.
What should I do if I get this 200-301 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 200-301 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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