Question 1hardmultiple choice
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Question 1,087 of 1,052
hardmultiple choiceObjective-mapped
CCNA Practice Question: Is troubleshooting an OSPFv2 adjacency issue…
This 200-301 practice question tests your understanding of 200-301 exam topics. 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 192.168.1.2 1 INIT/EXCHANGE 00:00:35 10.1.1.2 GigabitEthernet0/0 R1# show running-config | section router ospf 1 router ospf 1 router-id 192.168.1.1 network 10.1.1.0 0.0.0.255 area 0 passive-interface default no passive-interface GigabitEthernet0/0 R1# show ip interface GigabitEthernet0/0 | include OSPF OSPF is enabled, address 10.1.1.1, area 0, attched via network statement OSPF process ID 1, Router ID 192.168.1.1, Network Type BROADCAST, Cost: 1 OSPF timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5 OSPF is not passive on this interface
A network engineer is troubleshooting an OSPFv2 adjacency issue between two directly connected Cisco routers, R1 and R2. Both routers are configured in area 0, and the link between them is up/up. However, the OSPF neighbor state on R1 shows INIT/EXCHANGE and never reaches FULL. Based on the provided output from R1, what is the most likely cause of this problem?
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.
Clue:
"never"Why it matters: Absolute qualifier. True only if the statement has zero exceptions — be cautious of options that seem obvious but break down in edge cases.
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
✓
The MTU is mismatched between the two routers.
The OSPF neighbor state INIT/EXCHANGE indicates that R1 has received a hello from R2 but the adjacency is stuck during the database exchange phase. The most common cause for this is an MTU mismatch between the two routers. If R2 has a smaller MTU on its interface, it will reject large Database Description (DBD) packets from R1, causing the adjacency to stall in EXCHANGE/EXSTART. The output shows no obvious configuration errors in network statements, passive-interface, or router-id, and the neighbor is visible, so the issue is likely at a lower layer. Verifying and matching the MTU on both ends (e.g., using 'show ip interface') and adjusting it if necessary will resolve the issue.
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 router-id is mismatched between R1 and R2.
Why it's wrong here
The router-id is a unique identifier for each router in the OSPF domain. A mismatch does not prevent adjacency formation; it only affects the DR/BDR election if both have the same priority. The neighbor state shows R2's router-id (192.168.1.2) correctly, so this is not the issue.
- ✓
The MTU is mismatched between the two routers.
Why this is correct
An MTU mismatch causes OSPF to fail to exchange DBD packets, leaving the adjacency stuck in EXCHANGE/EXSTART state. This is a classic cause of the INIT/EXCHANGE state seen here. The solution is to ensure both interfaces have the same MTU value.
Clue confirmation
The clue words "most likely", "never" in the question point toward this answer.
Related concept
OSPF neighbours must agree on key parameters.
- ✗
The network statement does not match the interface IP address.
Why it's wrong here
The 'show ip interface' output confirms that OSPF is enabled on GigabitEthernet0/0 via a network statement covering 10.1.1.0/24. Since the interface IP is 10.1.1.1, it is correctly matched. An incorrect network statement would prevent OSPF from even seeing the neighbor.
- ✗
The passive-interface default command is blocking OSPF hellos on GigabitEthernet0/0.
Why it's wrong here
Although 'passive-interface default' is configured, the 'no passive-interface GigabitEthernet0/0' command overrides it for that interface. The output confirms OSPF is not passive on that interface, so hellos are sent and received.
Option-by-option analysis
Why each answer is right or wrong
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.
✓The MTU is mismatched between the two routers.Correct answer▾
Why this is correct
An MTU mismatch causes OSPF to fail to exchange DBD packets, leaving the adjacency stuck in EXCHANGE/EXSTART state. This is a classic cause of the INIT/EXCHANGE state seen here. The solution is to ensure both interfaces have the same MTU value.
✗The router-id is mismatched between R1 and R2.Wrong answer — click to see why▾
Why this is wrong here
Router-id mismatch does not cause INIT/EXCHANGE state; it is used for identification and election purposes.
✗The network statement does not match the interface IP address.Wrong answer — click to see why▾
Why this is wrong here
The network statement is correctly configured; OSPF is enabled and the neighbor is visible, so this is not the cause.
✗The passive-interface default command is blocking OSPF hellos on GigabitEthernet0/0.Wrong answer — click to see why▾
Why this is wrong here
The interface is explicitly set to not be passive, so this is not blocking the adjacency.
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
The router-id is a unique identifier for each router in the OSPF domain. A mismatch does not prevent adjacency formation; it only affects the DR/BDR election if both have the same priority. The neighbor state shows R2's router-id (192.168.1.2) correctly, so this is not the issue.
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.
What to study next
Got this wrong? Here's your next step.
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.
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FAQ
Questions learners often ask
What does this 200-301 question test?
OSPF neighbours must agree on key parameters.
What is the correct answer to this question?
The correct answer is: The MTU is mismatched between the two routers. — The OSPF neighbor state INIT/EXCHANGE indicates that R1 has received a hello from R2 but the adjacency is stuck during the database exchange phase. The most common cause for this is an MTU mismatch between the two routers. If R2 has a smaller MTU on its interface, it will reject large Database Description (DBD) packets from R1, causing the adjacency to stall in EXCHANGE/EXSTART. The output shows no obvious configuration errors in network statements, passive-interface, or router-id, and the neighbor is visible, so the issue is likely at a lower layer. Verifying and matching the MTU on both ends (e.g., using 'show ip interface') and adjusting it if necessary will resolve the issue.
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", "never". 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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