The answer is an OSPF area ID mismatch. This is the correct choice because OSPF requires that two routers forming a neighbor adjacency on the same link must agree on the area ID configured on that interface. Even though R1 and R2 have interfaces that are up/up and belong to the same subnet, R1’s interface is in area 0 while R2’s is in area 1, so the Hello packets they exchange carry different area IDs, causing the adjacency to fail before it can form. On the CCNA 200-301 v2 exam, this question tests your understanding of the OSPF neighbor state machine and the mandatory consistency checks in the ExStart state; a common trap is assuming that Layer 2 connectivity or a shared subnet alone guarantees OSPF adjacency. Remember the memory tip: "Same area, same story" — for OSPF neighbors to talk, their area IDs must match on the connecting link.
CCNA IP Routing Practice Question
This 200-301 practice question tests your understanding of ip routing. Examine the command output carefully: the correct answer depends on what the output actually shows, not on general recall alone. A key principle to apply: oSPF routers must be configured in the same area ID on directly connected interfaces to establish neighbor adjacency.. 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 G0/0: 10.12.12.1/30, ip ospf 1 area 0
R2 G0/0: 10.12.12.2/30, ip ospf 1 area 1
Both interfaces are up/up.
R1 and R2 are directly connected. Their interfaces are up/up and belong to the same subnet. R1's OSPF configuration places the interface in area 0, while R2's interface is in area 1. R1 does not show R2 as an OSPF neighbor. 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
✓
The area IDs do not match
OSPF neighbors on the same link must agree on the area ID. Here, one side is in area 0 and the other is in area 1, so adjacency never forms even though the interfaces are up and in the same subnet.
Key principle: OSPF routers must be configured in the same area ID on directly connected interfaces to establish neighbor adjacency.
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 area IDs do not match
Why this is correct
An area mismatch prevents the neighbor relationship.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
OSPF routers must be configured in the same area ID on directly connected interfaces to establish neighbor adjacency.
✗
R1 must use a loopback as the router ID
Why it's wrong here
A loopback helps stability but is not required to form adjacency.
When this WOULD be correct
In a different scenario where the question specifies that OSPF requires a router ID to be explicitly set and that the router ID must be a loopback address, this option would be correct. For example, if the question stated that R1 must use a loopback interface for OSPF to establish neighbor relationships, then this option would apply.
✗
The subnet mask is too small for OSPF to work
Why it's wrong here
A /30 is perfectly valid on a point-to-point link.
When this WOULD be correct
In a different scenario, if a question specified that R1 and R2 are on the same subnet but have mismatched subnet masks that prevent them from being in the same broadcast domain, then a small subnet mask could indeed be the reason for OSPF not establishing a neighbor relationship.
✗
R2 should be configured as the DR manually
Why it's wrong here
DR election is not the issue here.
When this WOULD be correct
In a different scenario where the question specifies a multi-access network with multiple OSPF routers, and the exam asks about ensuring a specific router is elected as the DR, this option would be correct if the manual configuration of DR is required to meet specific network design criteria.
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 area IDs do not matchCorrect answer▾
Why this is correct
An area mismatch prevents the neighbor relationship.
✗R1 must use a loopback as the router IDWrong answer — click to see why▾
Why this is wrong here
OSPF does not require a loopback interface as the router ID; any IP address on an active interface can be used, or the router ID can be manually configured. A loopback is recommended for stability but is not mandatory for forming neighbor relationships.
★ When this WOULD be the correct answer
In a different scenario where the question specifies that OSPF requires a router ID to be explicitly set and that the router ID must be a loopback address, this option would be correct. For example, if the question stated that R1 must use a loopback interface for OSPF to establish neighbor relationships, then this option would apply.
Why candidates choose this
Students often hear that loopback interfaces are best practice for OSPF router IDs and may mistakenly believe they are required. This confusion arises because many study materials emphasize using loopbacks for stability, leading to the incorrect assumption that OSPF will not work without one.
✗The subnet mask is too small for OSPF to workWrong answer — click to see why▾
Why this is wrong here
A /30 subnet mask (255.255.255.252) provides two usable host addresses, which is perfectly sufficient for a point-to-point link between two routers. OSPF works correctly with any valid subnet mask, including /30.
★ When this WOULD be the correct answer
In a different scenario, if a question specified that R1 and R2 are on the same subnet but have mismatched subnet masks that prevent them from being in the same broadcast domain, then a small subnet mask could indeed be the reason for OSPF not establishing a neighbor relationship.
Why candidates choose this
Some students think OSPF requires a larger subnet (e.g., /24) because they confuse OSPF's network type requirements with other protocols or assume that a small subnet might cause issues. However, OSPF operates fine on point-to-point links with /30 masks.
✗R2 should be configured as the DR manuallyWrong answer — click to see why▾
Why this is wrong here
On a point-to-point link, OSPF does not use a Designated Router (DR) or Backup Designated Router (BDR); the neighbor relationship forms directly without election. Manually configuring a DR is unnecessary and would not affect adjacency formation.
★ When this WOULD be the correct answer
In a different scenario where the question specifies a multi-access network with multiple OSPF routers, and the exam asks about ensuring a specific router is elected as the DR, this option would be correct if the manual configuration of DR is required to meet specific network design criteria.
Why candidates choose this
Students may recall that OSPF uses DR/BDR on multiaccess networks (like Ethernet) and incorrectly assume that DR election is always required. They might think that without a DR, OSPF cannot form neighbors, but this only applies to broadcast or NBMA networks, not point-to-point links.
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 common trap is focusing on router ID or subnet mask instead of recognizing that mismatched area IDs prevent OSPF adjacency.
Detailed technical explanation
How to think about this question
Open Shortest Path First (OSPF) is a link-state routing protocol that establishes neighbor relationships between routers on directly connected networks. These neighbors exchange routing information to build a consistent link-state database. A fundamental requirement for OSPF adjacency is that both routers must be in the same OSPF area, identified by the area ID. The area ID groups routers logically and controls the scope of link-state advertisements.
When two routers are directly connected, they send OSPF hello packets to discover neighbors. These hello packets include critical parameters such as the area ID, hello and dead intervals, and router IDs. If the area IDs do not match, the routers will not recognize each other as valid neighbors, and adjacency will fail to form. This is because OSPF treats each area as a separate routing domain, and routers in different areas do not form neighbor relationships on the same link.
A common exam trap is to assume that router ID configuration or subnet mask size causes adjacency failure. While router IDs are important for OSPF operation, they do not prevent neighbor formation if area IDs mismatch. Similarly, subnet masks like /30 are valid for point-to-point links and do not block adjacency. The Designated Router election also occurs after adjacency formation and does not prevent neighbor establishment. In practice, ensuring matching area IDs on connected interfaces is the first step to successful OSPF neighbor relationships.
KKey Concepts to Remember
OSPF routers must be configured in the same area ID on directly connected interfaces to establish neighbor adjacency.
OSPF forms neighbor relationships by exchanging hello packets that include area ID, router ID, and subnet information.
An area ID mismatch prevents OSPF routers from recognizing each other as neighbors, even if interfaces are up and in the same subnet.
OSPF router IDs can be manually set or automatically chosen, but using a loopback interface as router ID is not mandatory for adjacency.
OSPF point-to-point links support subnet masks as small as /30 without impacting neighbor formation.
The Designated Router (DR) election process does not affect the initial formation of OSPF neighbor relationships.
OSPF adjacency requires matching hello and dead intervals, area IDs, and subnet connectivity to successfully form neighbor states.
OSPF routers discard hello packets from neighbors with differing area IDs, preventing adjacency formation.
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 routers must be configured in the same area ID on directly connected interfaces to establish neighbor adjacency.
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 routers must be configured in the same area ID on directly connected interfaces to establish neighbor adjacency., then practise related 200-301 questions on the same topic to reinforce the concept.
IP Routing — This question tests IP Routing — OSPF routers must be configured in the same area ID on directly connected interfaces to establish neighbor adjacency..
What is the correct answer to this question?
The correct answer is: The area IDs do not match — OSPF neighbors on the same link must agree on the area ID. Here, one side is in area 0 and the other is in area 1, so adjacency never forms even though the interfaces are up and in the same subnet.
What should I do if I get this 200-301 question wrong?
Review oSPF routers must be configured in the same area ID on directly connected interfaces to establish neighbor adjacency., 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 routers must be configured in the same area ID on directly connected interfaces to establish neighbor adjacency.
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. Two OSPF routers connected over Ethernet fail to become neighbors. Their interfaces are up/up and in the same IPv4 subnet. One router uses area 0 and the other uses area 1 on the connecting interfaces. What is the most likely cause?
hard
A.Mismatched OSPF process IDs
✓ B.Mismatched OSPF areas on the shared link
C.Missing default routes
D.Different router IDs
Why B: OSPF neighbors on the same link must agree on key parameters, including the area assigned to that interface. A mismatch prevents the adjacency from forming.
Last reviewed: May 17, 2026
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