The answer is an OSPF hello and dead timer mismatch, specifically R1 using Hello 10/Dead 40 while R2 uses Hello 5/Dead 20, which prevents adjacency from forming. OSPF requires that both routers on a link agree on these timers because the hello interval determines how often keepalive packets are sent, and the dead interval dictates how long a neighbor is considered down without receiving one—if they differ, the routers will reject each other’s hellos and remain stuck in the INIT or EXSTART state. On the CCNA 200-301 v2 exam, this tests your ability to troubleshoot OSPF neighbor relationships, often appearing as a scenario where a single misconfigured interface blocks full adjacency despite correct network statements and router-ids. A common trap is assuming a router-id mismatch or passive-interface on the wrong link is the culprit, but remember that OSPF timers must match exactly on broadcast and point-to-point links. Memory tip: “Hello and Dead must be wed—if they don’t match, the neighbor’s dead.”
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.
Network Topology
You are connected to R1 via console. You have attempted to configure single-area OSPFv2 on R1 and R2 so that they form a full adjacency, but the adjacency is not establishing. The link between them is 192.168.1.0/30. R1 uses G0/0 (192.168.1.1/30) and R2 uses G0/0 (192.168.1.2/30). R1's router-id must be 1.1.1.1, and R2's router-id must be 2.2.2.2. Additionally, ensure that no OSPF hello packets are sent on R1's loopback0 interface (10.0.0.1/32). After troubleshooting, identify what is causing the issue and what must be corrected. Then verify the adjacency is full and passive-interface is set correctly.
R1#show running-config | section router ospf
router ospf 1
router-id 1.1.1.1
passive-interface loopback0
network 10.0.0.1 0.0.0.0 area 0
network 192.168.1.0 0.0.0.3 area 0
!
R1#show ip ospf interface gigabitethernet0/0
GigabitEthernet0/0 is up, line protocol is up
Internet Address 192.168.1.1/30, Area 0
Process ID 1, Router ID 1.1.1.1, Network Type BROADCAST, Cost: 1
Transmit Delay is 1 sec, State DR, Priority 1
Designated Router (ID) 1.1.1.1, Interface address 192.168.1.1
Backup Designated router (ID) 0.0.0.0, Interface address 0.0.0.0
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
Index 1/1, flood queue length 0
Next 0x0(0)/0x0(0)
Last flood scan length is 1, maximum is 1
Last flood scan time is 0 msec, maximum is 0 msec
Neighbor Count is 0, Adjacent neighbor count is 0
Suppress hello for 0 neighbor(s)
R1#show ip ospf neighbor
R1#
R2#show running-config | section router ospf
router ospf 1
router-id 2.2.2.2
network 192.168.1.0 0.0.0.3 area 0
!
R2#show ip ospf interface gigabitethernet0/0
GigabitEthernet0/0 is up, line protocol is up
Internet Address 192.168.1.2/30, Area 0
Process ID 1, Router ID 2.2.2.2, Network Type BROADCAST, Cost: 1
Transmit Delay is 1 sec, State DR, Priority 1
Designated Router (ID) 2.2.2.2, Interface address 192.168.1.2
Backup Designated router (ID) 0.0.0.0, Interface address 0.0.0.0
Timer intervals configured, Hello 5, Dead 20, Wait 20, Retransmit 5
oob-resync timeout 20
Hello due in 00:00:01
Supports Link-local Signaling (LLS)
Cisco NSF helper support enabled
IETF NSF helper support enabled
Index 1/1, flood queue length 0
Next 0x0(0)/0x0(0)
Last flood scan length is 1, maximum is 1
Last flood scan time is 0 msec, maximum is 0 msec
Neighbor Count is 0, Adjacent neighbor count is 0
Suppress hello for 0 neighbor(s)
A
The adjacency is not forming because the hello and dead timers are mismatched: R1 uses Hello 10 / Dead 40, but R2 uses Hello 5 / Dead 20. To fix, on R1's G0/0 interface, change the hello timer to 5 and dead timer to 20 using 'ip ospf hello-interval 5' and 'ip ospf dead-interval 20' (or adjust R2 to match R1's timers). After correction, the adjacency should become full. Additionally, R1's loopback0 is already correctly configured as passive-interface.
This is correct because OSPF requires matching hello and dead timers on both ends of a link to form an adjacency. The default timers on Cisco routers are Hello 10 seconds and Dead 40 seconds for broadcast networks, but R2 has been configured with non-default timers (Hello 5, Dead 20). Adjusting R1's timers to match R2's (or vice versa) resolves the mismatch. The passive-interface command on loopback0 prevents hello packets from being sent, which is correctly applied.
B
The adjacency is not forming because R1's router-id is not set correctly. To fix, configure 'router-id 1.1.1.1' under the OSPF process on R1, and 'router-id 2.2.2.2' on R2. After that, clear the OSPF process with 'clear ip ospf process' to force the new router-id to take effect. Additionally, R1's loopback0 is already correctly configured as passive-interface.
Why wrong: This is incorrect because the router-id configuration is not the issue; the problem is timer mismatch. The router-ids are correctly set as specified in the question. Even if they were not, router-id mismatch does not prevent adjacency formation as long as they are unique.
C
The adjacency is not forming because the network type is mismatched: R1 is configured as point-to-point, while R2 is broadcast. To fix, change the network type on both interfaces to the same type, e.g., 'ip ospf network point-to-point' on both. Additionally, R1's loopback0 is already correctly configured as passive-interface.
Why wrong: This is incorrect because the question does not indicate any network type mismatch. The default network type for Ethernet interfaces is broadcast, and both are likely broadcast. Network type mismatch can cause adjacency issues, but it is not the problem here; the timer mismatch is the culprit.
D
The adjacency is not forming because the passive-interface command is incorrectly applied to R1's G0/0 interface instead of loopback0. To fix, remove passive-interface from G0/0 using 'no passive-interface g0/0' and ensure it is only applied to loopback0. Additionally, verify that the router-ids are correctly set.
Why wrong: This is incorrect because the passive-interface command is correctly applied to loopback0, not G0/0. If passive-interface were applied to G0/0, no hello packets would be sent, preventing adjacency. However, the question states that the adjacency is not forming due to timer mismatch, not passive-interface misconfiguration.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The adjacency is not forming because the hello and dead timers are mismatched: R1 uses Hello 10 / Dead 40, but R2 uses Hello 5 / Dead 20. To fix, on R1's G0/0 interface, change the hello timer to 5 and dead timer to 20 using 'ip ospf hello-interval 5' and 'ip ospf dead-interval 20' (or adjust R2 to match R1's timers). After correction, the adjacency should become full. Additionally, R1's loopback0 is already correctly configured as passive-interface.
The correct answer is A: the adjacency is not forming because the hello and dead timers are mismatched. R1 uses Hello 10 / Dead 40, while R2 uses Hello 5 / Dead 20. To fix, on R1's G0/0 interface, change the hello timer to 5 and dead timer to 20 using 'ip ospf hello-interval 5' and 'ip ospf dead-interval 20' (or adjust R2 to match R1's timers). After correction, the adjacency becomes full. The other options are incorrect: B is wrong because router-ids can be set after the process starts and a mismatch does not prevent adjacency; C is wrong because network type mismatch would still allow adjacency if both agree (or if one is point-to-point and the other broadcast they will not form, but that is not the case here); D is wrong because the passive-interface on G0/0 would prevent hello packets and thus adjacency, but the question states it is only on loopback0, so that is not the issue. Verify with 'show ip ospf neighbor' and 'show ip ospf interface loopback0'.
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 adjacency is not forming because the hello and dead timers are mismatched: R1 uses Hello 10 / Dead 40, but R2 uses Hello 5 / Dead 20. To fix, on R1's G0/0 interface, change the hello timer to 5 and dead timer to 20 using 'ip ospf hello-interval 5' and 'ip ospf dead-interval 20' (or adjust R2 to match R1's timers). After correction, the adjacency should become full. Additionally, R1's loopback0 is already correctly configured as passive-interface.
Why this is correct
This is correct because OSPF requires matching hello and dead timers on both ends of a link to form an adjacency. The default timers on Cisco routers are Hello 10 seconds and Dead 40 seconds for broadcast networks, but R2 has been configured with non-default timers (Hello 5, Dead 20). Adjusting R1's timers to match R2's (or vice versa) resolves the mismatch. The passive-interface command on loopback0 prevents hello packets from being sent, which is correctly applied.
The adjacency is not forming because R1's router-id is not set correctly. To fix, configure 'router-id 1.1.1.1' under the OSPF process on R1, and 'router-id 2.2.2.2' on R2. After that, clear the OSPF process with 'clear ip ospf process' to force the new router-id to take effect. Additionally, R1's loopback0 is already correctly configured as passive-interface.
Why it's wrong here
This is incorrect because the router-id configuration is not the issue; the problem is timer mismatch. The router-ids are correctly set as specified in the question. Even if they were not, router-id mismatch does not prevent adjacency formation as long as they are unique.
✗
The adjacency is not forming because the network type is mismatched: R1 is configured as point-to-point, while R2 is broadcast. To fix, change the network type on both interfaces to the same type, e.g., 'ip ospf network point-to-point' on both. Additionally, R1's loopback0 is already correctly configured as passive-interface.
Why it's wrong here
This is incorrect because the question does not indicate any network type mismatch. The default network type for Ethernet interfaces is broadcast, and both are likely broadcast. Network type mismatch can cause adjacency issues, but it is not the problem here; the timer mismatch is the culprit.
✗
The adjacency is not forming because the passive-interface command is incorrectly applied to R1's G0/0 interface instead of loopback0. To fix, remove passive-interface from G0/0 using 'no passive-interface g0/0' and ensure it is only applied to loopback0. Additionally, verify that the router-ids are correctly set.
Why it's wrong here
This is incorrect because the passive-interface command is correctly applied to loopback0, not G0/0. If passive-interface were applied to G0/0, no hello packets would be sent, preventing adjacency. However, the question states that the adjacency is not forming due to timer mismatch, not passive-interface misconfiguration.
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 adjacency is not forming because the hello and dead timers are mismatched: R1 uses Hello 10 / Dead 40, but R2 uses Hello 5 / Dead 20. To fix, on R1's G0/0 interface, change the hello timer to 5 and dead timer to 20 using 'ip ospf hello-interval 5' and 'ip ospf dead-interval 20' (or adjust R2 to match R1's timers). After correction, the adjacency should become full. Additionally, R1's loopback0 is already correctly configured as passive-interface.Correct answer▾
Why this is correct
This is correct because OSPF requires matching hello and dead timers on both ends of a link to form an adjacency. The default timers on Cisco routers are Hello 10 seconds and Dead 40 seconds for broadcast networks, but R2 has been configured with non-default timers (Hello 5, Dead 20). Adjusting R1's timers to match R2's (or vice versa) resolves the mismatch. The passive-interface command on loopback0 prevents hello packets from being sent, which is correctly applied.
✗The adjacency is not forming because R1's router-id is not set correctly. To fix, configure 'router-id 1.1.1.1' under the OSPF process on R1, and 'router-id 2.2.2.2' on R2. After that, clear the OSPF process with 'clear ip ospf process' to force the new router-id to take effect. Additionally, R1's loopback0 is already correctly configured as passive-interface.Wrong answer — click to see why▾
Why this is wrong here
The specific factual error is that router-id mismatch does not cause adjacency failure; OSPF can form adjacency with different router-ids as long as they are unique. The question states the router-ids are already configured correctly.
Why candidates choose this
Candidates often think router-id must be explicitly set and that any mismatch causes issues. They may also confuse router-id with other OSPF parameters that must match.
✗The adjacency is not forming because the network type is mismatched: R1 is configured as point-to-point, while R2 is broadcast. To fix, change the network type on both interfaces to the same type, e.g., 'ip ospf network point-to-point' on both. Additionally, R1's loopback0 is already correctly configured as passive-interface.Wrong answer — click to see why▾
Why this is wrong here
The specific factual error is that there is no evidence of network type mismatch. The question focuses on timer mismatch, and network type is not mentioned as a configuration change.
Why candidates choose this
Candidates may recall that OSPF network types must match for adjacency, and they might jump to this conclusion without checking timers first. Network type mismatch is a common exam topic.
✗The adjacency is not forming because the passive-interface command is incorrectly applied to R1's G0/0 interface instead of loopback0. To fix, remove passive-interface from G0/0 using 'no passive-interface g0/0' and ensure it is only applied to loopback0. Additionally, verify that the router-ids are correctly set.Wrong answer — click to see why▾
Why this is wrong here
The specific factual error is that passive-interface is correctly configured on loopback0. The problem is timer mismatch, not passive-interface on the wrong interface.
Why candidates choose this
Candidates often confuse where passive-interface should be applied. They might think that if adjacency is not forming, it must be because hello packets are blocked, leading them to suspect passive-interface on the link interface.
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
This is incorrect because the passive-interface command is correctly applied to loopback0, not G0/0. If passive-interface were applied to G0/0, no hello packets would be sent, preventing adjacency. However, the question states that the adjacency is not forming due to timer mismatch, not passive-interface misconfiguration.
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: The adjacency is not forming because the hello and dead timers are mismatched: R1 uses Hello 10 / Dead 40, but R2 uses Hello 5 / Dead 20. To fix, on R1's G0/0 interface, change the hello timer to 5 and dead timer to 20 using 'ip ospf hello-interval 5' and 'ip ospf dead-interval 20' (or adjust R2 to match R1's timers). After correction, the adjacency should become full. Additionally, R1's loopback0 is already correctly configured as passive-interface. — The correct answer is A: the adjacency is not forming because the hello and dead timers are mismatched. R1 uses Hello 10 / Dead 40, while R2 uses Hello 5 / Dead 20. To fix, on R1's G0/0 interface, change the hello timer to 5 and dead timer to 20 using 'ip ospf hello-interval 5' and 'ip ospf dead-interval 20' (or adjust R2 to match R1's timers). After correction, the adjacency becomes full. The other options are incorrect: B is wrong because router-ids can be set after the process starts and a mismatch does not prevent adjacency; C is wrong because network type mismatch would still allow adjacency if both agree (or if one is point-to-point and the other broadcast they will not form, but that is not the case here); D is wrong because the passive-interface on G0/0 would prevent hello packets and thus adjacency, but the question states it is only on loopback0, so that is not the issue. Verify with 'show ip ospf neighbor' and 'show ip ospf interface loopback0'.
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.
What is the key concept behind this question?
OSPF neighbours must agree on key parameters.
About these practice questions
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These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. You are connected to R1. Configure single-area OSPFv2 on R1 and R2 so that they become fully adjacent. The link between them is 10.0.0.0/30, with R1 using G0/0 and R2 using G0/1. The current configuration has mismatched hello/dead timers: R1's G0/0 uses hello 10 and dead 40, while R2's G0/1 uses hello 30 and dead 120. Also, the 'passive-interface loopback0' command is missing on R1. Ensure OSPF is enabled in area 0, use router-id 1.1.1.1 on R1 and 2.2.2.2 on R2, and correct the timer mismatch.
hard
✓ A.On R2, configure 'ip ospf hello-interval 10' and 'ip ospf dead-interval 40' under G0/1. On R1, configure 'passive-interface loopback0' under router ospf.
B.On R1, configure 'ip ospf hello-interval 30' and 'ip ospf dead-interval 120' under G0/0. On R2, configure 'passive-interface loopback0' under router ospf.
C.On R1, configure 'ip ospf hello-interval 10' and 'ip ospf dead-interval 40' under G0/0. On R2, configure 'passive-interface loopback0' under router ospf.
D.On R2, configure 'ip ospf hello-interval 30' and 'ip ospf dead-interval 120' under G0/1. On R1, configure 'passive-interface loopback0' under router ospf.
Why A: The adjacency fails because R2's G0/1 has hello/dead timers of 30/120, while R1's G0/0 uses 10/40. To fix, on R2 under interface G0/1, use 'ip ospf hello-interval 10' and 'ip ospf dead-interval 40'. Additionally, R1's loopback0 is advertised via a /32 network statement but should be made passive to prevent OSPF hellos from being sent out (where no neighbor exists). On R1, configure 'passive-interface loopback0' under router ospf. After both changes, adjacency forms.
Last reviewed: Jun 6, 2026
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