The answer is an OSPF authentication key mismatch. This is the most likely cause because OSPF neighbor formation requires a complete handshake where all configured parameters must match exactly, including authentication credentials. Even when Layer 3 connectivity is verified via ping, and critical parameters like area IDs and Hello/Dead timers are identical, mismatched authentication keys cause OSPF to silently discard all incoming Hello packets, preventing any adjacency from forming. On the CCNA 200-301 v2 exam, this scenario tests your understanding that OSPF authentication operates as a separate, mandatory check in the neighbor state machine—a common trap is assuming that successful pings or matching timers guarantee adjacency. Remember that OSPF authentication is often overlooked because it is configured under the interface or area, not in the main OSPF process. A useful memory tip is “Ping is not a neighbor”: connectivity does not equal adjacency, and authentication is the silent killer of OSPF relationships.
CCNA IP Routing Practice Question
This 200-301 practice question tests your understanding of ip routing. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. A key principle to apply: oSPF routers must have matching authentication keys and types to successfully form neighbor adjacencies.. 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#
interface GigabitEthernet0/0
ip address 10.0.12.1 255.255.255.252
ip ospf authentication message-digest
ip ospf message-digest-key 1 md5 cisco123
!
router ospf 1
network 10.0.12.0 0.0.0.3 area 0
R2#
interface GigabitEthernet0/0
ip address 10.0.12.2 255.255.255.252
ip ospf authentication message-digest
ip ospf message-digest-key 1 md5 cisco321
!
router ospf 1
network 10.0.12.0 0.0.0.3 area 0
R1 and R2 are directly connected and running OSPF. They can ping each other, the area matches, and the timers match, but they still do not become neighbors. 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 OSPF authentication keys do not match.
The most likely cause is a mismatch in OSPF authentication keys. Even though the routers have IP connectivity, matching area IDs, and identical timers, OSPF adjacency requires that authentication parameters also match. If authentication is enabled on both sides but the keys differ, OSPF packets are silently rejected, preventing neighbor formation.
Key principle: OSPF routers must have matching authentication keys and types to successfully form neighbor adjacencies.
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 OSPF authentication keys do not match.
Why this is correct
This is correct because mismatched OSPF MD5 authentication prevents neighbor formation even when addressing and area values are correct.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
OSPF routers must have matching authentication keys and types to successfully form neighbor adjacencies.
✗
The subnet mask is too small for OSPF to operate.
Why it's wrong here
This is wrong because OSPF works normally on /30 links.
When this WOULD be correct
In a different scenario where the question states that R1 and R2 are configured with OSPF but are on different subnets due to a misconfigured subnet mask, leading to a lack of reachability, this option would be correct. For example, if R1 is on 192.168.1.0/30 and R2 is on 192.168.1.4/30, they cannot form an OSPF neighbor relationship.
✗
The routers must use different process IDs.
Why it's wrong here
This is wrong because process IDs are locally significant and do not need to match or differ.
When this WOULD be correct
In a different scenario, if the question stated that R1 and R2 are in the same area but have different OSPF process IDs, then this option would be correct, as OSPF routers must have the same process ID to form a neighbor relationship.
✗
The interfaces must be configured as switch trunks.
Why it's wrong here
This is wrong because OSPF commonly runs over routed interfaces.
When this WOULD be correct
In a different scenario where the question specifies that R1 and R2 are connected via a switch and need to establish OSPF over multiple VLANs, the interfaces would need to be configured as switch trunks to allow OSPF traffic across those VLANs. If the trunking is misconfigured, OSPF neighbors would not form.
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 OSPF authentication keys do not match.Correct answer▾
Why this is correct
This is correct because mismatched OSPF MD5 authentication prevents neighbor formation even when addressing and area values are correct.
✗The subnet mask is too small for OSPF to operate.Wrong answer — click to see why▾
Why this is wrong here
OSPF does not require a specific subnet mask size; it works on any subnet, including /30 links. The issue described (ping working but OSPF not forming adjacency) is not caused by a subnet mask being too small.
★ When this WOULD be the correct answer
In a different scenario where the question states that R1 and R2 are configured with OSPF but are on different subnets due to a misconfigured subnet mask, leading to a lack of reachability, this option would be correct. For example, if R1 is on 192.168.1.0/30 and R2 is on 192.168.1.4/30, they cannot form an OSPF neighbor relationship.
Why candidates choose this
Students may confuse OSPF's requirement for matching subnet masks on the same link (which is true for OSPF network type broadcast) with the mask being too small. However, a /30 mask is perfectly valid and commonly used for point-to-point links.
✗The routers must use different process IDs.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. Two routers can have different process IDs and still form a neighbor adjacency as long as other parameters (area, timers, authentication) match.
★ When this WOULD be the correct answer
In a different scenario, if the question stated that R1 and R2 are in the same area but have different OSPF process IDs, then this option would be correct, as OSPF routers must have the same process ID to form a neighbor relationship.
Why candidates choose this
Students often confuse OSPF process IDs with EIGRP autonomous system numbers, which must match. This leads to the mistaken belief that OSPF process IDs must also match.
✗The interfaces must be configured as switch trunks.Wrong answer — click to see why▾
Why this is wrong here
OSPF runs on routed interfaces (Layer 3), not on switch trunks (Layer 2). Configuring interfaces as switch trunks would prevent OSPF from operating because trunks are used for VLAN tagging, not for routing protocols.
★ When this WOULD be the correct answer
In a different scenario where the question specifies that R1 and R2 are connected via a switch and need to establish OSPF over multiple VLANs, the interfaces would need to be configured as switch trunks to allow OSPF traffic across those VLANs. If the trunking is misconfigured, OSPF neighbors would not form.
Why candidates choose this
Some students may think that OSPF requires trunking because they have seen OSPF configured on VLAN interfaces or subinterfaces, but that is different from configuring a physical interface as a trunk port.
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 overlook OSPF authentication mismatches when routers have IP connectivity and matching area IDs. Candidates may incorrectly assume that because the routers can ping each other and timers match, adjacency must form. However, if OSPF authentication keys differ, routers silently reject OSPF packets, preventing neighbor formation. This trap exploits the misconception that IP reachability alone ensures OSPF adjacency, ignoring the critical role of matching authentication parameters in the OSPF neighbor negotiation process.
Detailed technical explanation
How to think about this question
Open Shortest Path First (OSPF) is a link-state routing protocol that forms neighbor adjacencies to exchange routing information. For two routers to become OSPF neighbors, they must share several key parameters: the same area ID, matching hello and dead intervals, and identical authentication settings if authentication is enabled. Authentication in OSPF can be simple password or message-digest (MD5), which secures OSPF packets by requiring matching keys on both routers. Without matching authentication keys, OSPF packets are discarded, preventing neighbor formation.
The OSPF adjacency formation process involves routers exchanging hello packets to discover neighbors and negotiate parameters. If any critical parameter differs, such as area ID or authentication key, the routers will not progress beyond the init or two-way state. Process IDs are locally significant and do not need to match; they only identify the OSPF process on a router. Additionally, OSPF runs over routed interfaces and does not require switch trunking, which is relevant for VLAN tagging but unrelated to OSPF neighbor formation.
A common exam trap is assuming that IP connectivity and matching area IDs guarantee OSPF adjacency. However, when authentication is enabled, mismatched keys cause silent failures where routers can ping each other but never become neighbors. This scenario highlights the importance of verifying all OSPF parameters, especially authentication keys, during troubleshooting. In practical networks, mismatched MD5 keys are a frequent cause of adjacency failures despite correct IP and area configurations.
KKey Concepts to Remember
OSPF routers must have matching authentication keys and types to successfully form neighbor adjacencies.
OSPF adjacency formation requires identical area IDs, hello and dead timers, and authentication parameters on both routers.
OSPF process IDs are locally significant and do not affect neighbor relationships between routers.
OSPF can operate over routed point-to-point links without requiring switch trunk configurations on interfaces.
Mismatched OSPF authentication keys cause routers to reject each other's OSPF packets despite successful IP connectivity.
OSPF adjacency depends on protocol parameter matching beyond basic IP reachability, including authentication and area configuration.
OSPF message-digest (MD5) authentication provides secure neighbor relationships by requiring matching keys on both ends.
IP connectivity alone does not guarantee OSPF neighbor formation if protocol parameters such as authentication keys differ.
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 have matching authentication keys and types to successfully form neighbor adjacencies.
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 have matching authentication keys and types to successfully form neighbor adjacencies., then practise related 200-301 questions on the same topic to reinforce the concept.
IP Routing — This question tests IP Routing — OSPF routers must have matching authentication keys and types to successfully form neighbor adjacencies..
What is the correct answer to this question?
The correct answer is: The OSPF authentication keys do not match. — The most likely cause is a mismatch in OSPF authentication keys. Even though the routers have IP connectivity, matching area IDs, and identical timers, OSPF adjacency requires that authentication parameters also match. If authentication is enabled on both sides but the keys differ, OSPF packets are silently rejected, preventing neighbor formation.
What should I do if I get this 200-301 question wrong?
Review oSPF routers must have matching authentication keys and types to successfully form neighbor adjacencies., 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 have matching authentication keys and types to successfully form neighbor adjacencies.
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