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 running-config | section router ospf
router ospf 1
router-id 1.1.1.1
network 203.0.113.0 0.0.0.3 area 0
network 203.0.113.128 0.0.0.0 area 0
passive-interface Loopback0
R1# show ip ospf neighbor
R1# show ip ospf interface gigabitethernet 0/0
GigabitEthernet0/0 is up, line protocol is up
Internet Address 203.0.113.1/30, Area 0
Process ID 1, Router ID 1.1.1.1, Network Type BROADCAST, Cost: 1
Transmit Delay is 1 sec, State WAITING, Priority 1
No Hellos (Passive interface)
R2# show running-config | section router ospf
router ospf 1
router-id 2.2.2.2
network 203.0.113.0 0.0.0.3 area 0
You are connected to R1 via the console. Configure single-area OSPFv2 on R1 and R2 so that they form a full adjacency. The link between R1 and R2 uses 203.0.113.0/30. R1 has G0/0 203.0.113.1/30 and R2 has G0/0 203.0.113.2/30. R1's router-id must be 1.1.1.1, and R2's router-id must be 2.2.2.2. R1's GigabitEthernet0/0 interface is configured as a passive interface under OSPF, preventing OSPF hello messages from being sent out of that interface. Ensure that R1 does not send OSPF hellos out of its loopback0 interface (203.0.113.129/32). After configuration, verify the adjacency is established and OSPF routes are exchanged.
R1# show running-config | section router ospf
router ospf 1
router-id 1.1.1.1
network 203.0.113.0 0.0.0.3 area 0
network 203.0.113.128 0.0.0.0 area 0
passive-interface Loopback0
R1# show ip ospf neighbor
R1# show ip ospf interface gigabitethernet 0/0
GigabitEthernet0/0 is up, line protocol is up
Internet Address 203.0.113.1/30, Area 0
Process ID 1, Router ID 1.1.1.1, Network Type BROADCAST, Cost: 1
Transmit Delay is 1 sec, State WAITING, Priority 1
No Hellos (Passive interface)
R2# show running-config | section router ospf
router ospf 1
router-id 2.2.2.2
network 203.0.113.0 0.0.0.3 area 0
A
The adjacency fails because R1's GigabitEthernet0/0 interface is configured as passive-interface. Remove the passive-interface command for G0/0.
This is correct because a passive-interface in OSPF suppresses hello packets, preventing neighbor discovery and adjacency formation. The loopback0 should remain passive, but G0/0 must send hellos to establish adjacency with R2.
B
The adjacency fails because the router-id 1.1.1.1 is not reachable from R2. Configure a static route for 1.1.1.1/32 on R2.
Why wrong: This is incorrect because router-ids are used for OSPF router identification and do not need to be reachable via routing. They are selected from the highest loopback or physical IP, or manually set.
C
The adjacency fails because the subnet mask on the link is /30 but OSPF expects a /24. Change the mask to /24 on both interfaces.
Why wrong: This is incorrect because OSPF does not require a specific subnet mask; it can form adjacency over any mask as long as the interfaces are in the same subnet. A /30 is perfectly valid for point-to-point links.
D
The adjacency fails because OSPF is not enabled on R2's GigabitEthernet0/0 interface. Configure 'ip ospf 1 area 0' on R2's G0/0.
Why wrong: This is incorrect because the question implies both routers are configured for OSPF; the issue is specifically on R1's passive-interface. If OSPF were not enabled on R2's interface, there would be no hello packets at all, but the scenario shows R1 is not sending hellos.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The adjacency fails because R1's GigabitEthernet0/0 interface is configured as passive-interface. Remove the passive-interface command for G0/0.
The adjacency fails because R1's GigabitEthernet0/0 interface is configured as passive-interface (the 'No Hellos' line in show ip ospf interface). This prevents R1 from sending OSPF hellos to R2. To fix, remove the passive-interface command for G0/0. The loopback0 interface should remain passive. After removal, verify with 'show ip ospf neighbor' to see the neighbor state change to FULL and 'show ip route ospf' to see routes.
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 fails because R1's GigabitEthernet0/0 interface is configured as passive-interface. Remove the passive-interface command for G0/0.
Why this is correct
This is correct because a passive-interface in OSPF suppresses hello packets, preventing neighbor discovery and adjacency formation. The loopback0 should remain passive, but G0/0 must send hellos to establish adjacency with R2.
The adjacency fails because the router-id 1.1.1.1 is not reachable from R2. Configure a static route for 1.1.1.1/32 on R2.
Why it's wrong here
This is incorrect because router-ids are used for OSPF router identification and do not need to be reachable via routing. They are selected from the highest loopback or physical IP, or manually set.
✗
The adjacency fails because the subnet mask on the link is /30 but OSPF expects a /24. Change the mask to /24 on both interfaces.
Why it's wrong here
This is incorrect because OSPF does not require a specific subnet mask; it can form adjacency over any mask as long as the interfaces are in the same subnet. A /30 is perfectly valid for point-to-point links.
✗
The adjacency fails because OSPF is not enabled on R2's GigabitEthernet0/0 interface. Configure 'ip ospf 1 area 0' on R2's G0/0.
Why it's wrong here
This is incorrect because the question implies both routers are configured for OSPF; the issue is specifically on R1's passive-interface. If OSPF were not enabled on R2's interface, there would be no hello packets at all, but the scenario shows R1 is not sending hellos.
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 fails because R1's GigabitEthernet0/0 interface is configured as passive-interface. Remove the passive-interface command for G0/0.Correct answer▾
Why this is correct
This is correct because a passive-interface in OSPF suppresses hello packets, preventing neighbor discovery and adjacency formation. The loopback0 should remain passive, but G0/0 must send hellos to establish adjacency with R2.
✗The adjacency fails because the router-id 1.1.1.1 is not reachable from R2. Configure a static route for 1.1.1.1/32 on R2.Wrong answer — click to see why▾
Why this is wrong here
The specific factual error is that router-ids are not required to be routable; they are merely identifiers within the OSPF domain.
Why candidates choose this
Candidates might think that since router-id is an IP address, it must be reachable for OSPF to work, confusing it with a loopback interface used for BGP or other protocols.
✗The adjacency fails because the subnet mask on the link is /30 but OSPF expects a /24. Change the mask to /24 on both interfaces.Wrong answer — click to see why▾
Why this is wrong here
The specific factual error is that OSPF has no mask requirement beyond matching subnet; /30 is commonly used for serial links.
Why candidates choose this
Candidates might recall that some routing protocols (like RIPv2) have issues with certain masks, or confuse OSPF network types (point-to-point vs broadcast) with mask requirements.
✗The adjacency fails because OSPF is not enabled on R2's GigabitEthernet0/0 interface. Configure 'ip ospf 1 area 0' on R2's G0/0.Wrong answer — click to see why▾
Why this is wrong here
The specific factual error is that the problem is on R1's side, not R2's. R2's interface is likely enabled but not receiving hellos from R1.
Why candidates choose this
Candidates often check both sides for OSPF enablement and might assume R2 is misconfigured, especially if they see 'show ip ospf neighbor' showing nothing.
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 question implies both routers are configured for OSPF; the issue is specifically on R1's passive-interface. If OSPF were not enabled on R2's interface, there would be no hello packets at all, but the scenario shows R1 is not sending hellos.
Scenario analysis trap
This is incorrect because the question implies both routers are configured for OSPF; the issue is specifically on R1's passive-interface. If OSPF were not enabled on R2's interface, there would be no hello packets at all, but the scenario shows R1 is not sending hellos.
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 fails because R1's GigabitEthernet0/0 interface is configured as passive-interface. Remove the passive-interface command for G0/0. — The adjacency fails because R1's GigabitEthernet0/0 interface is configured as passive-interface (the 'No Hellos' line in show ip ospf interface). This prevents R1 from sending OSPF hellos to R2. To fix, remove the passive-interface command for G0/0. The loopback0 interface should remain passive. After removal, verify with 'show ip ospf neighbor' to see the neighbor state change to FULL and 'show ip route ospf' to see routes.
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.
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