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Question 71 of 1,052
hardmultiple choiceObjective-mapped
CCNA Practice Question: An engineer is troubleshooting an OSPF adjacency…
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
2.2.2.2 1 EXSTART/EXCHANGE 00:00:35 10.1.1.2 GigabitEthernet0/0
R1# show ip ospf interface gigabitEthernet 0/0
GigabitEthernet0/0 is up, line protocol is up
Internet Address 10.1.1.1/24, Area 0, Attached via Network Statement
Process ID 1, Router ID 1.1.1.1, Network Type BROADCAST, Cost: 1
Topology-MTID Cost Disabled Shutdown Topology Name
0 1 no no Base
Transmit Delay is 1 sec, State DR, Priority 1
Designated Router (ID) 1.1.1.1, Interface address 10.1.1.1
Backup Designated router (ID) 2.2.2.2, Interface address 10.1.1.2
Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
oob-resync timeout 40
Hello due in 00:00:03
Supports Link-local Signaling (LLS)
Cisco NSF helper support enabled
IETF NSF helper support enabled
Can be used as a virtual-link endpoint
Index 1/1/1, runqueue 0x0
Neighbor Count is 1, Adjacent neighbor count is 0
Adjacent with neighbor 2.2.2.2 (Backup Designated Router)
Suppress hello for 0 neighbor(s)
R1# show running-config | section router ospf
router ospf 1
router-id 1.1.1.1
network 10.1.1.0 0.0.0.255 area 0
passive-interface default
no passive-interface GigabitEthernet0/0An engineer is troubleshooting an OSPF adjacency issue between two Cisco routers, R1 and R2, connected via GigabitEthernet0/0 on both sides. Hosts on R1's LAN cannot ping hosts on R2's LAN. The engineer checks the OSPF neighbor state on R1 and sees the adjacency is stuck in EXSTART/EXCHANGE. The router IDs are 1.1.1.1 on R1 and 2.2.2.2 on R2, and both routers have a network statement for their directly connected subnet. 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.
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
✓
There is an MTU mismatch between R1 and R2 on the GigabitEthernet0/0 interfaces.
The adjacency is stuck in EXSTART/EXCHANGE, which typically indicates an MTU mismatch between the two OSPF neighbors. In this scenario, R1 has an MTU of 1500 bytes on its GigabitEthernet0/0 (default), but the exhibit does not show the MTU on R2. However, the most common cause for EXSTART/EXCHANGE state is MTU mismatch, as OSPF Database Description (DBD) packets are dropped if they exceed the interface MTU. The other options (passive-interface, area mismatch, router-id conflict) would cause different adjacency states or no adjacency at all. For example, a passive-interface would prevent Hello packets from being sent, resulting in no neighbor, not EXSTART/EXCHANGE. An area mismatch would cause the neighbor to be stuck in INIT or 2WAY. A router-id conflict would cause the neighbor to be stuck in EXSTART with constant retransmissions, but the exhibit shows a single neighbor, so conflict is unlikely.
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 passive-interface default command is blocking OSPF Hellos on GigabitEthernet0/0.
Why it's wrong here
The 'no passive-interface GigabitEthernet0/0' command under router ospf ensures Hellos are sent on that interface, so this is not the issue.
- ✓
There is an MTU mismatch between R1 and R2 on the GigabitEthernet0/0 interfaces.
Why this is correct
An MTU mismatch causes OSPF DBD packets to be dropped, leading to the EXSTART/EXCHANGE state. Fixing the MTU on one side to match the other resolves the adjacency.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
OSPF neighbours must agree on key parameters.
- ✗
The OSPF area configured on the interface does not match between R1 and R2.
Why it's wrong here
Both routers have area 0 configured for the 10.1.1.0/24 network, as shown in the running config and exhibit, so area mismatch is not the issue.
- ✗
The router IDs 1.1.1.1 and 2.2.2.2 are conflicting with each other.
Why it's wrong here
Router IDs must be unique; they are different here (1.1.1.1 vs 2.2.2.2), so no conflict exists.
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.
✓There is an MTU mismatch between R1 and R2 on the GigabitEthernet0/0 interfaces.Correct answer▾
Why this is correct
An MTU mismatch causes OSPF DBD packets to be dropped, leading to the EXSTART/EXCHANGE state. Fixing the MTU on one side to match the other resolves the adjacency.
✗The passive-interface default command is blocking OSPF Hellos on GigabitEthernet0/0.Wrong answer — click to see why▾
Why this is wrong here
A passive interface would prevent Hellos from being sent, resulting in no neighbor adjacency at all, not EXSTART/EXCHANGE.
✗The OSPF area configured on the interface does not match between R1 and R2.Wrong answer — click to see why▾
Why this is wrong here
An area mismatch would cause the neighbor to be stuck in INIT or 2WAY, not EXSTART/EXCHANGE.
✗The router IDs 1.1.1.1 and 2.2.2.2 are conflicting with each other.Wrong answer — click to see why▾
Why this is wrong here
A router ID conflict would cause the neighbor to be stuck in EXSTART with constant retransmissions, but the exhibit shows a single neighbor with no mention of duplicate IDs.
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 'no passive-interface GigabitEthernet0/0' command under router ospf ensures Hellos are sent on that interface, 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: There is an MTU mismatch between R1 and R2 on the GigabitEthernet0/0 interfaces. — The adjacency is stuck in EXSTART/EXCHANGE, which typically indicates an MTU mismatch between the two OSPF neighbors. In this scenario, R1 has an MTU of 1500 bytes on its GigabitEthernet0/0 (default), but the exhibit does not show the MTU on R2. However, the most common cause for EXSTART/EXCHANGE state is MTU mismatch, as OSPF Database Description (DBD) packets are dropped if they exceed the interface MTU. The other options (passive-interface, area mismatch, router-id conflict) would cause different adjacency states or no adjacency at all. For example, a passive-interface would prevent Hello packets from being sent, resulting in no neighbor, not EXSTART/EXCHANGE. An area mismatch would cause the neighbor to be stuck in INIT or 2WAY. A router-id conflict would cause the neighbor to be stuck in EXSTART with constant retransmissions, but the exhibit shows a single neighbor, so conflict is unlikely.
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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