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 route ospf
Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2
i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
ia - IS-IS inter area, * - candidate default, U - per-user static route
o - ODR, P - periodic downloaded static route, H - NHRP, l - LISP
a - application route
+ - replicated route, % - next hop override
Gateway of last resort is not set
192.168.10.0/24 is variably subnetted, 2 subnets, 2 masks
O 192.168.10.0/24 [110/2] via 10.1.1.2, 00:35:14, GigabitEthernet0/1
192.168.20.0/24 is variably subnetted, 2 subnets, 2 masks
O 192.168.20.0/24 [110/2] via 10.1.1.2, 00:35:14, GigabitEthernet0/1
R1#
Refer to the exhibit. A network engineer is troubleshooting a missing route on R1. The router R3 is configured to advertise network 192.168.30.0/24 via OSPF, but the route is not present in the OSPF routing table of R1 when issuing the show ip route ospf command. What is the most likely cause?
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 route ospf
Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2
i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
ia - IS-IS inter area, * - candidate default, U - per-user static route
o - ODR, P - periodic downloaded static route, H - NHRP, l - LISP
a - application route
+ - replicated route, % - next hop override
Gateway of last resort is not set
192.168.10.0/24 is variably subnetted, 2 subnets, 2 masks
O 192.168.10.0/24 [110/2] via 10.1.1.2, 00:35:14, GigabitEthernet0/1
192.168.20.0/24 is variably subnetted, 2 subnets, 2 masks
O 192.168.20.0/24 [110/2] via 10.1.1.2, 00:35:14, GigabitEthernet0/1
R1#
A
R3 has not enabled OSPF on the network 192.168.30.0/24 using the network command.
Why wrong: This would also cause the route to be absent, but the scenario already states that R3 is configured to advertise the network, so the missing network statement is not the cause.
B
R3 has passive-interface default configured and has not issued the no passive-interface command on the interface facing the OSPF neighbor.
Passive-interface default suppresses OSPF hello packets on all interfaces unless explicitly enabled. Without hellos, an adjacency cannot form, so R1 never learns the 192.168.30.0/24 route. The exhibit shows OSPF working with another neighbor, confirming the problem is isolated to R3’s interface configuration.
C
R1 has an inbound ACL applied to GigabitEthernet0/1 that blocks OSPF multicast traffic to 224.0.0.5 and 224.0.0.6.
Why wrong: If such an ACL existed, R1 would not be able to form any OSPF adjacency on that interface, yet the exhibit shows R1 has learned 192.168.10.0/24 and 192.168.20.0/24 via the same GigabitEthernet0/1. This proves OSPF multicast traffic is being received and processed correctly.
D
R3's OSPF router ID is duplicated with another OSPF router, causing the SPF algorithm to drop routes.
Why wrong: A duplicate router ID can cause adjacency flapping or failure to establish, but if hellos are still exchanged, an adjacency might form temporarily. However, this issue would typically affect all routes from that router, and the exhibit shows no route from R3 at all, which points more directly to an adjacency never forming because hellos were never sent (passive-interface). Moreover, the OSPF neighbor table would show the neighbor in a problematic state, but we are given only the routing table; the passive-interface scenario cleanly explains the missing route without conflicting with observable facts.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
R3 has passive-interface default configured and has not issued the no passive-interface command on the interface facing the OSPF neighbor.
The exhibit from R1# show ip route ospf displays OSPF routes for 192.168.10.0/24 and 192.168.20.0/24 learned via 10.1.1.2 on GigabitEthernet0/1, proving that OSPF is functioning with neighbor R2. The route 192.168.30.0/24 that R3 is supposed to advertise is missing, indicating that R1 has not formed an OSPF adjacency with R3. When 'passive-interface default' is configured, OSPF hello packets are suppressed on all interfaces unless explicitly overridden with 'no passive-interface'. As a result, R3 does not send hellos, preventing adjacency and route advertisement. This directly explains why the exhibit lacks the 192.168.30.0/24 entry.
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.
✗
R3 has not enabled OSPF on the network 192.168.30.0/24 using the network command.
Why it's wrong here
This would also cause the route to be absent, but the scenario already states that R3 is configured to advertise the network, so the missing network statement is not the cause.
✓
R3 has passive-interface default configured and has not issued the no passive-interface command on the interface facing the OSPF neighbor.
Why this is correct
Passive-interface default suppresses OSPF hello packets on all interfaces unless explicitly enabled. Without hellos, an adjacency cannot form, so R1 never learns the 192.168.30.0/24 route. The exhibit shows OSPF working with another neighbor, confirming the problem is isolated to R3’s interface configuration.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
R1 has an inbound ACL applied to GigabitEthernet0/1 that blocks OSPF multicast traffic to 224.0.0.5 and 224.0.0.6.
Why it's wrong here
If such an ACL existed, R1 would not be able to form any OSPF adjacency on that interface, yet the exhibit shows R1 has learned 192.168.10.0/24 and 192.168.20.0/24 via the same GigabitEthernet0/1. This proves OSPF multicast traffic is being received and processed correctly.
✗
R3's OSPF router ID is duplicated with another OSPF router, causing the SPF algorithm to drop routes.
Why it's wrong here
A duplicate router ID can cause adjacency flapping or failure to establish, but if hellos are still exchanged, an adjacency might form temporarily. However, this issue would typically affect all routes from that router, and the exhibit shows no route from R3 at all, which points more directly to an adjacency never forming because hellos were never sent (passive-interface). Moreover, the OSPF neighbor table would show the neighbor in a problematic state, but we are given only the routing table; the passive-interface scenario cleanly explains the missing route without conflicting with observable facts.
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.
✓R3 has passive-interface default configured and has not issued the no passive-interface command on the interface facing the OSPF neighbor.Correct answer▾
Why this is correct
Passive-interface default suppresses OSPF hello packets on all interfaces unless explicitly enabled. Without hellos, an adjacency cannot form, so R1 never learns the 192.168.30.0/24 route. The exhibit shows OSPF working with another neighbor, confirming the problem is isolated to R3’s interface configuration.
✗R3 has not enabled OSPF on the network 192.168.30.0/24 using the network command.Wrong answer — click to see why▾
Why this is wrong here
The question says R3 advertises the network via OSPF; a missing network command would contradict that, so the failure must be in adjacency formation.
✗R1 has an inbound ACL applied to GigabitEthernet0/1 that blocks OSPF multicast traffic to 224.0.0.5 and 224.0.0.6.Wrong answer — click to see why▾
Why this is wrong here
The presence of other OSPF routes on the same interface disproves a blanket multicast ACL; the issue is specific to R3’s adjacency.
✗R3's OSPF router ID is duplicated with another OSPF router, causing the SPF algorithm to drop routes.Wrong answer — click to see why▾
Why this is wrong here
A duplicate router ID could prevent adjacency, but the scenario’s focus on passive-interface default provides a more common and direct cause for a total lack of adjacency when OSPF is correctly configured on the network.
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
If such an ACL existed, R1 would not be able to form any OSPF adjacency on that interface, yet the exhibit shows R1 has learned 192.168.10.0/24 and 192.168.20.0/24 via the same GigabitEthernet0/1. This proves OSPF multicast traffic is being received and processed correctly.
Scenario analysis trap
This would also cause the route to be absent, but the scenario already states that R3 is configured to advertise the network, so the missing network statement is not the cause.
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: R3 has passive-interface default configured and has not issued the no passive-interface command on the interface facing the OSPF neighbor. — The exhibit from R1# show ip route ospf displays OSPF routes for 192.168.10.0/24 and 192.168.20.0/24 learned via 10.1.1.2 on GigabitEthernet0/1, proving that OSPF is functioning with neighbor R2. The route 192.168.30.0/24 that R3 is supposed to advertise is missing, indicating that R1 has not formed an OSPF adjacency with R3. When 'passive-interface default' is configured, OSPF hello packets are suppressed on all interfaces unless explicitly overridden with 'no passive-interface'. As a result, R3 does not send hellos, preventing adjacency and route advertisement. This directly explains why the exhibit lacks the 192.168.30.0/24 entry.
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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