The answer is that R1’s GigabitEthernet0/0 is configured as a passive OSPF interface, which prevents neighbor adjacency. This is correct because a passive interface stops the router from sending OSPF hello packets out that link, so R2 never receives the hello needed to form a neighbor relationship. Even though the network statement includes the subnet, OSPF still requires active hello exchange to discover neighbors; a passive interface advertises the connected route but blocks adjacency formation. On the CCNA 200-301 v2 exam, this is a classic troubleshooting trap—candidates often assume a correct network statement guarantees adjacency, but the passive-interface command silently breaks neighbor discovery. The key memory tip is: passive means “no hello, no neighbor,” so always check for it when OSPF routes are missing despite correct subnet and area configuration.
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. A key principle to apply: oSPF uses hello packets on interfaces to discover and maintain neighbor adjacencies necessary for route exchange.. 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#
router ospf 1
network 10.20.12.0 0.0.0.255 area 0
passive-interface GigabitEthernet0/0
!
interface GigabitEthernet0/0
ip address 10.20.12.1 255.255.255.0
R2#
router ospf 1
network 10.20.12.0 0.0.0.255 area 0
!
interface GigabitEthernet0/0
ip address 10.20.12.2 255.255.255.0
R1 and R2 are connected via a GigabitEthernet link in the same IPv4 subnet, and both routers have OSPF configured in the same area. However, R1 is not learning any OSPF routes from R2. 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.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
GigabitEthernet0/0 is configured as a passive OSPF interface on R1.
The most likely reason is that one side has the interface configured as passive, which prevents OSPF hello packets from being sent on that interface. In practical terms, the network statement alone does not guarantee neighbor formation. OSPF still needs active neighbor discovery on the link. If the interface is passive, the router advertises the connected network into OSPF but does not attempt to form an adjacency there.
This is a realistic routing troubleshooting pattern because the configuration can look mostly correct until you inspect the passive-interface setting.
Key principle: OSPF uses hello packets on interfaces to discover and maintain neighbor adjacencies necessary for route exchange.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✓
GigabitEthernet0/0 is configured as a passive OSPF interface on R1.
Why this is correct
This is correct because a passive OSPF interface does not send hellos and therefore will not form a neighbor adjacency on that link.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
OSPF uses hello packets on interfaces to discover and maintain neighbor adjacencies necessary for route exchange.
✗
The routers must use different OSPF process IDs to exchange routes.
Why it's wrong here
This is wrong because OSPF process IDs are locally significant and do not need to match.
When this WOULD be correct
In a different scenario where the question specifies that R1 and R2 are configured with different OSPF process IDs, this option would be correct. For example, if the question asked why R1 cannot learn routes from R2 when both routers are configured with distinct OSPF process IDs, this option would accurately explain the situation.
✗
The routers are in different IPv4 subnets.
Why it's wrong here
This is wrong because both interfaces are in 10.20.12.0/24.
When this WOULD be correct
In a different scenario, if the question stated that R1 and R2 are configured in separate OSPF areas and are indeed in different IPv4 subnets, this option would be correct. The question would need to focus on the inability of OSPF to exchange routes across different subnets without proper routing protocols or configurations.
✗
OSPF can advertise routes only across serial links.
Why it's wrong here
This is wrong because OSPF commonly operates across Ethernet networks.
When this WOULD be correct
In a different exam scenario where the question specifies that OSPF is only configured on serial interfaces and the routers are connected via Ethernet, this option could be correct. The question would need to clarify that OSPF is restricted to serial links due to specific network design constraints.
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.
✓GigabitEthernet0/0 is configured as a passive OSPF interface on R1.Correct answer▾
Why this is correct
This is correct because a passive OSPF interface does not send hellos and therefore will not form a neighbor adjacency on that link.
✗The routers must use different OSPF process IDs to exchange routes.Wrong answer — click to see why▾
Why this is wrong here
OSPF process IDs are locally significant and do not need to match between routers for neighbor adjacency to form. The process ID only identifies the OSPF process on the local router, so different process IDs do not prevent route exchange.
★ When this WOULD be the correct answer
In a different scenario where the question specifies that R1 and R2 are configured with different OSPF process IDs, this option would be correct. For example, if the question asked why R1 cannot learn routes from R2 when both routers are configured with distinct OSPF process IDs, this option would accurately explain the situation.
Why candidates choose this
Students often confuse OSPF process IDs with other routing protocols like EIGRP, where the autonomous system number must match. They may incorrectly assume that OSPF process IDs must be consistent across routers.
✗The routers are in different IPv4 subnets.Wrong answer — click to see why▾
Why this is wrong here
The exhibit shows both interfaces are in the same subnet (10.20.12.0/24), so subnet mismatch is not the issue. OSPF requires interfaces to be in the same subnet to form a neighbor adjacency.
★ When this WOULD be the correct answer
In a different scenario, if the question stated that R1 and R2 are configured in separate OSPF areas and are indeed in different IPv4 subnets, this option would be correct. The question would need to focus on the inability of OSPF to exchange routes across different subnets without proper routing protocols or configurations.
Why candidates choose this
Subnet mismatch is a common OSPF neighbor issue, and test-takers may jump to this conclusion without verifying the actual IP addresses and subnet masks in the exhibit.
✗OSPF can advertise routes only across serial links.Wrong answer — click to see why▾
Why this is wrong here
OSPF can operate over any type of IP network, including Ethernet, Frame Relay, and serial links. The statement that OSPF can only advertise routes across serial links is factually incorrect.
★ When this WOULD be the correct answer
In a different exam scenario where the question specifies that OSPF is only configured on serial interfaces and the routers are connected via Ethernet, this option could be correct. The question would need to clarify that OSPF is restricted to serial links due to specific network design constraints.
Why candidates choose this
Some older OSPF implementations or exam scenarios emphasize serial links, leading students to believe OSPF is limited to serial connections. However, OSPF is widely used on Ethernet networks.
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
A frequent exam trap is to incorrectly believe that OSPF process IDs must match between routers to exchange routes or that OSPF only works on serial links. Candidates may also overlook the passive-interface setting, assuming that the presence of correct network statements guarantees neighbor formation. The passive-interface command disables hello packets, which are essential for OSPF adjacency. This subtle configuration detail often causes confusion because the router still advertises the network but refuses to form neighbors, leading to missing routes despite seemingly correct OSPF setup.
Detailed technical explanation
How to think about this question
OSPF (Open Shortest Path First) is a link-state routing protocol that relies on the exchange of hello packets between routers on a shared network segment to establish neighbor adjacencies. These adjacencies are essential for routers to share link-state information and build a synchronized routing database. Without forming neighbor relationships, OSPF routers cannot exchange routing updates, preventing route learning.
The passive-interface command in OSPF configuration disables the sending of hello packets on the specified interface. While the router continues to advertise the connected network into OSPF, it does not attempt to form neighbor adjacencies on that interface. This is useful for interfaces connected to end devices or networks where no OSPF neighbors exist. However, if mistakenly applied on an interface connecting two OSPF routers, it prevents adjacency formation and route exchange.
A common exam trap involves assuming that matching OSPF process IDs or correct subnetting alone guarantees neighbor formation. In reality, the passive-interface setting overrides these factors by stopping hello packets. Practically, this means that even if network statements and IP addressing are correct, no routes will be learned if one side is passive. Understanding this behavior is critical for troubleshooting OSPF connectivity issues and ensuring proper routing in Cisco networks.
KKey Concepts to Remember
OSPF uses hello packets on interfaces to discover and maintain neighbor adjacencies necessary for route exchange.
Configuring an interface as passive in OSPF prevents the sending of hello packets, thus blocking neighbor adjacency formation on that interface.
OSPF process IDs are locally significant and do not need to match between routers to establish neighbor relationships.
OSPF requires interfaces to be in the same IP subnet to form neighbor adjacencies and exchange routing information.
OSPF operates over multiple link types including Ethernet and serial, and is not limited to serial links only.
A passive interface in OSPF still advertises its connected network but does not participate in neighbor discovery or route exchange on that link.
Neighbor adjacency formation in OSPF depends on active hello packet exchanges, which are disabled on passive interfaces.
Misconfiguring an interface as passive in OSPF is a common troubleshooting cause for missing routes despite correct network statements.
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
OSPF uses hello packets on interfaces to discover and maintain neighbor adjacencies necessary for route exchange.
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 uses hello packets on interfaces to discover and maintain neighbor adjacencies necessary for route exchange., then practise related 200-301 questions on the same topic to reinforce the concept.
IP Routing — This question tests IP Routing — OSPF uses hello packets on interfaces to discover and maintain neighbor adjacencies necessary for route exchange..
What is the correct answer to this question?
The correct answer is: GigabitEthernet0/0 is configured as a passive OSPF interface on R1. — The most likely reason is that one side has the interface configured as passive, which prevents OSPF hello packets from being sent on that interface. In practical terms, the network statement alone does not guarantee neighbor formation. OSPF still needs active neighbor discovery on the link. If the interface is passive, the router advertises the connected network into OSPF but does not attempt to form an adjacency there.
This is a realistic routing troubleshooting pattern because the configuration can look mostly correct until you inspect the passive-interface setting.
What should I do if I get this 200-301 question wrong?
Review oSPF uses hello packets on interfaces to discover and maintain neighbor adjacencies necessary for route exchange., then practise related 200-301 questions on the same topic to reinforce the concept.
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 uses hello packets on interfaces to discover and maintain neighbor adjacencies necessary for route exchange.
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