The answer is an OSPF authentication mismatch, specifically that the authentication key does not match on the two routers. When OSPF authentication is enabled on an interface, every Hello packet includes authentication data, and a router will reject any Hello that does not present the correct key; since both routers are on the same subnet and in the same area, the only remaining variable blocking adjacency is the mismatched security credential. On the CCNA 200-301 v2 exam, this scenario tests your ability to isolate Layer 3 adjacency issues from Layer 2 connectivity, and the common trap is assuming that simply enabling authentication is enough—both the key type (plaintext or MD5) and the key string must be identical on each side. A useful memory tip is “same key, same type, same trust” because OSPF neighbors will not exchange routing information until they authenticate each other’s identity.
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 use matching authentication keys on a shared segment 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 10.10.12.1 255.255.255.0
ip ospf authentication message-digest
ip ospf message-digest-key 1 md5 C1sc0
!
router ospf 1
network 10.10.12.0 0.0.0.255 area 0
R2#
interface GigabitEthernet0/0
ip address 10.10.12.2 255.255.255.0
ip ospf authentication message-digest
ip ospf message-digest-key 1 md5 C1scO
!
router ospf 1
network 10.10.12.0 0.0.0.255 area 0
R1 and R2 are connected via a shared Ethernet segment. Both routers are configured in OSPF area 0 and are on the same IP subnet. OSPF authentication is enabled on both interfaces, but the adjacency is not forming. What is the most likely reason?
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 key does not match on the two routers.
The most likely reason is an OSPF authentication mismatch. In practical terms, both routers are on the same IP subnet and in the same area, but they are not using the same authentication key on the shared link. OSPF neighbors must agree on key authentication parameters before they will trust each other enough to form an adjacency.
This is a classic CCNA troubleshooting pattern because the configuration looks almost correct until you compare the security settings carefully.
Key principle: OSPF routers must use matching authentication keys on a shared segment 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 key does not match on the two routers.
Why this is correct
This is correct because an authentication mismatch prevents OSPF adjacency on the shared segment.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
OSPF routers must use matching authentication keys on a shared segment to successfully form neighbor adjacencies.
✗
The routers must use different OSPF areas to become neighbors.
Why it's wrong here
This is wrong because neighbors on the same segment must agree on the area.
When this WOULD be correct
In a different scenario where the question specifies that R1 and R2 are configured in different OSPF areas, the option would be correct. For example, if the question stated that R1 is in area 0 and R2 is in area 1, this option would accurately reflect the requirement for OSPF adjacency.
✗
OSPF message-digest authentication can be used only on serial links.
Why it's wrong here
This is wrong because OSPF authentication can be used on Ethernet as well.
When this WOULD be correct
In a different question setup where the context specifies that OSPF message-digest authentication is only applicable to serial links, a scenario could involve two routers connected via a serial link attempting to authenticate with message-digest but failing due to incorrect configuration. In this case, the option would be correct.
✗
The routers must remove IP addressing before OSPF can form.
Why it's wrong here
This is wrong because the link needs valid IP addressing for OSPF adjacency.
When this WOULD be correct
In a different question, if it were stated that the routers were configured to use a specific OSPF feature that required no IP addressing for a unique setup, such as a lab environment simulating a non-IP OSPF scenario, then this option could be correct.
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 key does not match on the two routers.Correct answer▾
Why this is correct
This is correct because an authentication mismatch prevents OSPF adjacency on the shared segment.
✗The routers must use different OSPF areas to become neighbors.Wrong answer — click to see why▾
Why this is wrong here
This option is incorrect because OSPF can form adjacencies between routers in the same area; they do not need to be in different areas to become neighbors. OSPF requires routers to be in the same area to establish adjacency on a shared segment.
★ When this WOULD be the correct answer
In a different scenario where the question specifies that R1 and R2 are configured in different OSPF areas, the option would be correct. For example, if the question stated that R1 is in area 0 and R2 is in area 1, this option would accurately reflect the requirement for OSPF adjacency.
Why candidates choose this
Candidates may choose this option due to a misunderstanding of OSPF area configurations, believing that adjacency can only occur between routers in different areas, which is a common misconception among those new to OSPF.
✗OSPF message-digest authentication can be used only on serial links.Wrong answer — click to see why▾
Why this is wrong here
OSPF message-digest authentication is not limited to serial links; it can be implemented on any type of link, including Ethernet segments. Therefore, this option does not accurately address the reason for the adjacency failure between R1 and R2.
★ When this WOULD be the correct answer
In a different question setup where the context specifies that OSPF message-digest authentication is only applicable to serial links, a scenario could involve two routers connected via a serial link attempting to authenticate with message-digest but failing due to incorrect configuration. In this case, the option would be correct.
Why candidates choose this
Candidates may find this option tempting due to a misunderstanding of OSPF authentication types and their applicability, leading them to incorrectly associate message-digest authentication with specific link types.
✗The routers must remove IP addressing before OSPF can form.Wrong answer — click to see why▾
Why this is wrong here
This option is wrong because OSPF can form adjacencies with routers that have IP addresses configured on the same segment; removing IP addressing is not a requirement for OSPF operation.
★ When this WOULD be the correct answer
In a different question, if it were stated that the routers were configured to use a specific OSPF feature that required no IP addressing for a unique setup, such as a lab environment simulating a non-IP OSPF scenario, then this option could be correct.
Why candidates choose this
Candidates might choose this option due to a misunderstanding of OSPF requirements, confusing the need for proper IP addressing with the adjacency formation process, leading them to believe that removing IP addresses could somehow resolve adjacency issues.
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 selecting an answer that incorrectly states OSPF authentication is limited to serial links or that routers must be in different areas to form adjacency. Candidates may also mistakenly believe that removing IP addressing is necessary for OSPF to form. These misconceptions overlook that OSPF authentication applies to Ethernet interfaces and that neighbors must be in the same area and subnet with matching authentication keys. Misreading these details leads to choosing incorrect options that seem plausible but contradict OSPF adjacency rules.
Detailed technical explanation
How to think about this question
OSPF (Open Shortest Path First) is a link-state routing protocol that requires routers to form neighbor adjacencies before exchanging routing information. On a shared Ethernet segment, routers send Hello packets to discover neighbors and establish adjacency. These Hello packets must match on several parameters including area ID, subnet, and authentication settings. Authentication secures OSPF messages by requiring routers to use the same authentication method and key, ensuring only trusted routers participate in routing.
When OSPF authentication is enabled, routers include an authentication key in their Hello packets. If the keys do not match, the routers will not recognize each other as valid neighbors and will not progress beyond the Init state to form a full adjacency. This prevents routing information from being exchanged, which can cause connectivity issues. Cisco routers support multiple authentication types on Ethernet interfaces, including simple password and message digest (MD5), and these must be consistently configured.
A common exam trap is assuming that authentication is only relevant on serial links or that mismatched keys cause an IP addressing conflict. In reality, authentication mismatches silently block adjacency formation without generating explicit error messages. Practically, network engineers must verify that all OSPF parameters, especially authentication keys, match exactly on all routers in the same area and subnet to ensure successful adjacency and stable routing.
KKey Concepts to Remember
OSPF routers must use matching authentication keys on a shared segment to successfully form neighbor adjacencies.
OSPF adjacency formation requires routers to be in the same area and share identical network parameters including authentication settings.
OSPF authentication can be configured on any interface type, including Ethernet, to secure routing updates between neighbors.
A mismatch in OSPF authentication keys prevents the exchange of OSPF Hello packets, stopping adjacency formation.
OSPF neighbors must have compatible IP addressing on the shared segment to recognize each other and establish adjacency.
OSPF authentication parameters include the authentication type and the key, both of which must match exactly on all routers.
OSPF adjacency failures due to authentication mismatches are a common troubleshooting scenario in Cisco network environments.
Correct OSPF configuration on shared Ethernet segments includes consistent area, IP subnet, and authentication settings.
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 use matching authentication keys on a shared segment 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 use matching authentication keys on a shared segment 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 use matching authentication keys on a shared segment to successfully form neighbor adjacencies..
What is the correct answer to this question?
The correct answer is: The OSPF authentication key does not match on the two routers. — The most likely reason is an OSPF authentication mismatch. In practical terms, both routers are on the same IP subnet and in the same area, but they are not using the same authentication key on the shared link. OSPF neighbors must agree on key authentication parameters before they will trust each other enough to form an adjacency.
This is a classic CCNA troubleshooting pattern because the configuration looks almost correct until you compare the security settings carefully.
What should I do if I get this 200-301 question wrong?
Review oSPF routers must use matching authentication keys on a shared segment 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 use matching authentication keys on a shared segment to successfully form neighbor adjacencies.
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