The answer is an OSPF authentication mismatch. This is the most likely cause because OSPF neighbor formation requires that directly connected routers share identical authentication settings on their interfaces; R1 is configured with message-digest authentication while R2 has none, so their Hello packets are rejected before adjacency can form. On the CCNA 200-301 v2 exam, this scenario tests your understanding that OSPF authentication is a per-interface parameter that must match between neighbors, and it is a common trap to assume that Layer 3 reachability alone guarantees OSPF adjacency. A useful memory tip is “match or mismatch” — always verify that authentication type and key ID are identical on both sides, as even a single missing command breaks the neighbor relationship.
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 routers must have matching authentication settings on their interfaces 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 10.1.12.1 255.255.255.0
ip ospf authentication message-digest
ip ospf message-digest-key 1 md5 cisco123
!
router ospf 10
network 10.1.12.0 0.0.0.255 area 0
R2#
interface GigabitEthernet0/0
ip address 10.1.12.2 255.255.255.0
!
router ospf 10
network 10.1.12.0 0.0.0.255 area 0
R1 and R2 are directly connected. Both are configured in OSPF area 0, and they can successfully ping each other. However, OSPF neighbor adjacency fails. R1's interface is configured with `ip ospf authentication message-digest` and a valid key, while R2's interface has no OSPF authentication configured. 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 settings do not match.
The most likely cause is an OSPF authentication mismatch. Although the routers have IP connectivity and are in the same OSPF area, OSPF adjacency requires matching security parameters. R1 uses message-digest authentication whereas R2 has none configured, preventing neighbor formation. The other options are incorrect: the stem confirms they are in the same area (A), OSPF does not require identical hostnames (C), and successful pings prove the subnet mask does not block multicast traffic (D).
Key principle: OSPF routers must have matching authentication settings on their interfaces 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 routers are in different OSPF areas.
Why it's wrong here
This is wrong because both routers place the subnet into area 0.
When this WOULD be correct
In a different scenario, if the question indicated that R1 and R2 were configured in different OSPF areas, such as Area 0 for R1 and Area 1 for R2, then this option would be correct as routers in different areas cannot form OSPF adjacencies.
✓
The OSPF authentication settings do not match.
Why this is correct
This is correct because one side expects MD5 authentication and the other side is not shown with matching authentication.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
OSPF routers must have matching authentication settings on their interfaces to successfully form neighbor adjacencies.
✗
The routers need identical hostnames before adjacency can form.
Why it's wrong here
This is wrong because hostnames do not determine OSPF adjacency.
When this WOULD be correct
In a different scenario where the question specifies that OSPF adjacency is failing due to a requirement for routers to have matching hostnames for a proprietary implementation or a specific vendor's OSPF configuration, this option would be correct.
✗
The subnet mask prevents OSPF multicast traffic.
Why it's wrong here
This is wrong because the /24 mask does not block OSPF multicast here.
When this WOULD be correct
In a scenario where two routers are configured with different subnet masks on their directly connected interfaces, and the question specifically asks about OSPF adjacency issues related to subnet configurations, this option would be correct. For example, if one router is configured with a /24 mask and the other with a /30 mask, OSPF would fail to form an adjacency.
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 settings do not match.Correct answer▾
Why this is correct
This is correct because one side expects MD5 authentication and the other side is not shown with matching authentication.
✗The routers are in different OSPF areas.Wrong answer — click to see why▾
Why this is wrong here
The exhibit shows both routers place the subnet into area 0, so they are in the same OSPF area. Different areas would prevent adjacency, but that is not the case here.
★ When this WOULD be the correct answer
In a different scenario, if the question indicated that R1 and R2 were configured in different OSPF areas, such as Area 0 for R1 and Area 1 for R2, then this option would be correct as routers in different areas cannot form OSPF adjacencies.
Why candidates choose this
Students often confuse OSPF area mismatch as a common cause of adjacency failure, and if they misread the exhibit, they might think the areas differ.
✗The routers need identical hostnames before adjacency can form.Wrong answer — click to see why▾
Why this is wrong here
OSPF adjacency does not require identical hostnames; hostnames are only used for identification and do not affect the OSPF neighbor state machine.
★ When this WOULD be the correct answer
In a different scenario where the question specifies that OSPF adjacency is failing due to a requirement for routers to have matching hostnames for a proprietary implementation or a specific vendor's OSPF configuration, this option would be correct.
Why candidates choose this
Some students might confuse OSPF with protocols like EIGRP where the router ID must be unique, but hostnames are not a factor in OSPF adjacency formation.
✗The subnet mask prevents OSPF multicast traffic.Wrong answer — click to see why▾
Why this is wrong here
The /24 subnet mask does not block OSPF multicast traffic (224.0.0.5 and 224.0.0.6). OSPF multicasts are sent regardless of the subnet mask on the interface.
★ When this WOULD be the correct answer
In a scenario where two routers are configured with different subnet masks on their directly connected interfaces, and the question specifically asks about OSPF adjacency issues related to subnet configurations, this option would be correct. For example, if one router is configured with a /24 mask and the other with a /30 mask, OSPF would fail to form an adjacency.
Why candidates choose this
Students might think that a non-default subnet mask could affect multicast delivery, but OSPF uses link-local multicast addresses that are not filtered by the subnet mask.
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 assuming that successful ping and matching OSPF areas guarantee neighbor adjacency. Candidates often overlook OSPF authentication mismatches, especially when one router uses message-digest authentication and the other does not. This leads to confusion because the link appears operational at Layer 3, but OSPF packets are discarded silently. The trap exploits the misconception that IP connectivity alone is sufficient for OSPF adjacency, ignoring the protocol’s security requirements.
Detailed technical explanation
How to think about this question
Open Shortest Path First (OSPF) is a link-state routing protocol that requires routers to establish neighbor adjacencies before exchanging routing information. This process involves routers sending hello packets to discover and maintain neighbor relationships. For adjacency to form, routers must agree on several parameters including the OSPF area, hello and dead intervals, subnet mask, and authentication settings. Authentication ensures that only trusted routers participate in the OSPF domain, enhancing network security.
When OSPF authentication is enabled, routers use either simple password or message-digest (MD5) authentication to verify the identity of neighbors. Both routers on a link must have matching authentication types and keys configured; otherwise, they will reject each other's OSPF packets. Even if routers can ping each other and share the same area, a mismatch in authentication settings prevents neighbor adjacency formation because the OSPF protocol treats unauthenticated packets as invalid.
This authentication mismatch is a common exam trap because it is easy to overlook when basic IP connectivity and area configuration appear correct. In practice, routers will maintain Layer 3 connectivity but fail to exchange OSPF routing updates, causing routing issues. Understanding that OSPF adjacency depends on protocol-level agreement beyond IP reachability is critical for troubleshooting and exam success.
KKey Concepts to Remember
OSPF routers must have matching authentication settings on their interfaces to successfully form neighbor adjacencies.
OSPF adjacency formation requires agreement on area assignment, authentication, hello and dead intervals, and subnet mask consistency.
Basic IP connectivity like successful ping does not guarantee OSPF neighbor formation if protocol parameters differ.
OSPF authentication mismatches cause routers to discard OSPF packets, preventing neighbor relationships despite Layer 3 reachability.
OSPF message-digest authentication (MD5) requires both routers to configure identical keys and authentication types on shared links.
Router hostnames do not impact OSPF neighbor formation or adjacency establishment in any way.
Subnet masks influence IP routing and reachability but do not block OSPF multicast traffic if correctly configured.
OSPF neighbors must be in the same area and have matching interface parameters to exchange routing information and form adjacencies.
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 settings on their interfaces 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 settings on their interfaces 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 settings on their interfaces to successfully form neighbor adjacencies..
What is the correct answer to this question?
The correct answer is: The OSPF authentication settings do not match. — The most likely cause is an OSPF authentication mismatch. Although the routers have IP connectivity and are in the same OSPF area, OSPF adjacency requires matching security parameters. R1 uses message-digest authentication whereas R2 has none configured, preventing neighbor formation. The other options are incorrect: the stem confirms they are in the same area (A), OSPF does not require identical hostnames (C), and successful pings prove the subnet mask does not block multicast traffic (D).
What should I do if I get this 200-301 question wrong?
Review oSPF routers must have matching authentication settings on their interfaces 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 settings on their interfaces to successfully form neighbor adjacencies.
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