The answer is an MTU mismatch between R1 and R2 on the GigabitEthernet0/0 interfaces. This is correct because OSPF neighbors stuck in EXSTART/EXCHANGE have already passed the 2-Way state and are attempting to exchange Database Description (DBD) packets; when the interface MTU differs, the router with the smaller MTU drops the larger DBD packet, preventing the exchange from completing and leaving the adjacency permanently stuck in EXSTART. On the CCNA 200-301 v2 exam, this scenario tests your understanding of OSPF neighbor state progression and the specific role of DBD packets—a common trap is confusing this with a passive interface or area mismatch, which would block Hello packets and prevent reaching EXSTART at all. Remember that OSPF uses the interface MTU to determine DBD packet size, so if one side sends a 1500-byte DBD and the other only accepts 1400, the adjacency stalls. A helpful memory tip: "EXSTART needs DBDs to start, but a smaller MTU tears them apart."
CCNA IP Routing Practice Question
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
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 GigabitEthernet0/0
An 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.
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 GigabitEthernet0/0
A
The passive-interface default command is blocking OSPF Hellos on GigabitEthernet0/0.
Why wrong: The 'no passive-interface GigabitEthernet0/0' command under router ospf ensures Hellos are sent on that interface, so this is not the issue.
B
There is an MTU mismatch between R1 and R2 on the GigabitEthernet0/0 interfaces.
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.
C
The OSPF area configured on the interface does not match between R1 and R2.
Why wrong: 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.
D
The router IDs 1.1.1.1 and 2.2.2.2 are conflicting with each other.
Why wrong: Router IDs must be unique; they are different here (1.1.1.1 vs 2.2.2.2), so no conflict exists.
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 EXSTART/EXCHANGE state indicates that OSPF neighbors have progressed past the 2-Way state and are attempting to exchange Database Description (DBD) packets. An MTU mismatch on the directly connected GigabitEthernet0/0 interfaces prevents successful DBD exchange because the interface with the smaller MTU drops the larger DBD packet, causing the adjacency to remain stuck in EXSTART/EXCHANGE. Options A and C are incorrect because passive-interface default and area mismatches prevent the adjacency from reaching EXSTART/EXCHANGE (they block Hello exchange), and option D is incorrect because conflicting router IDs would prevent adjacency formation entirely, not result in a stuck EXSTART/EXCHANGE state.
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 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
Read the scenario before looking for a memorised answer.
✗
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
The passive-interface default command would suppress OSPF Hellos on all interfaces, preventing the adjacency from forming at all, not just getting stuck in EXSTART/EXCHANGE. The adjacency would remain in DOWN or INIT state, not EXSTART/EXCHANGE.
Why candidates choose this
Students may confuse the effect of passive-interface with causing adjacency issues, but the specific state EXSTART/EXCHANGE indicates that Hellos have been exchanged and the routers are trying to synchronize databases, which requires DBD packets to be sent.
✗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 prevent the adjacency from forming at all, typically resulting in the neighbor state remaining in INIT or 2-WAY, not EXSTART/EXCHANGE. The question states both routers have a network statement for the directly connected subnet, implying the same area is configured.
Why candidates choose this
Area mismatch is a common OSPF issue, but it manifests earlier in the adjacency process. Students might assume any adjacency problem is due to area mismatch without considering the specific state.
✗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
Router IDs must be unique for OSPF to form an adjacency. Here, the router IDs are different (1.1.1.1 and 2.2.2.2), so there is no conflict. A conflict would cause the adjacency to fail, but not specifically stuck in EXSTART/EXCHANGE.
Why candidates choose this
Students may think that router IDs need to be in the same subnet or similar, but OSPF only requires uniqueness. The specific state EXSTART/EXCHANGE points to a DBD packet issue, not a router ID conflict.
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: answer the scenario, not the keyword
Cisco often tests the MTU mismatch scenario by presenting symptoms of a stuck adjacency in EXSTART/EXCHANGE, which candidates may confuse with other issues like mismatched areas or passive interfaces, but the key clue is the specific state where DBD exchange fails.
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
In OSPF, the MTU value is advertised in the Database Description (DBD) packet's interface MTU field. If the receiving router's interface has a smaller MTU, it will reject the DBD packet, causing the neighbor to stay in EXSTART/EXCHANGE. This behavior is defined in RFC 2328, and the 'ip ospf mtu-ignore' command can be used to bypass this check in troubleshooting scenarios, though it is not a best practice for production networks.
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.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 200-301 question in full detail.
IP Routing — This question tests IP Routing — Read the scenario before looking for a memorised answer..
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 EXSTART/EXCHANGE state indicates that OSPF neighbors have progressed past the 2-Way state and are attempting to exchange Database Description (DBD) packets. An MTU mismatch on the directly connected GigabitEthernet0/0 interfaces prevents successful DBD exchange because the interface with the smaller MTU drops the larger DBD packet, causing the adjacency to remain stuck in EXSTART/EXCHANGE. Options A and C are incorrect because passive-interface default and area mismatches prevent the adjacency from reaching EXSTART/EXCHANGE (they block Hello exchange), and option D is incorrect because conflicting router IDs would prevent adjacency formation entirely, not result in a stuck EXSTART/EXCHANGE state.
What should I do if I get this 200-301 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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