CCNA IP Routing Practice Question
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?
⚠ Common exam trap
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.
Answer choices
Why each option matters
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.
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
OSPF interfaces configured with an authentication key must match the key and key ID on neighboring routers. When R1 sends a Hello packet, it computes an authentication hash using its configured key; R2 verifies that value with its own key. A mismatch causes the receiving router to silently drop the Hello, so no adjacency can form even though all other OSPF parameters are correct.
- ✗
The routers must use different OSPF areas to become neighbors.
Why it's wrong here
To become OSPF neighbors on the same segment, the routers must have the exact same OSPF area ID configured for that link, not different areas. The Area ID field is carried inside the OSPF Hello packet, and a receiver immediately rejects a Hello with a mismatched area number. A correct configuration would use one shared area, such as area 0, for both routers.
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
OSPF MD5 message-digest authentication is not tied to any particular interface type; it is a configurable security feature that works on Ethernet, point-to-point serial, and other OSPF-enabled interfaces. The network administrator may choose authentication type 'message-digest' on any interface, and the restriction in this option does not exist. Therefore, saying it can only be used on serial links is factually incorrect.
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
Removing IP addressing from an interface would make OSPF unable to form an adjacency because OSPF requires the interface to have a valid IPv4 or IPv6 address to originate and receive Hello packets. The source IP address in each Hello is used for neighbor identification and next-hop reachability. Without an assigned address, the OSPF interface state remains down, and no adjacency can ever be established.
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
OSPF interfaces configured with an authentication key must match the key and key ID on neighboring routers. When R1 sends a Hello packet, it computes an authentication hash using its configured key; R2 verifies that value with its own key. A mismatch causes the receiving router to silently drop the Hello, so no adjacency can form even though all other OSPF parameters are correct.
✗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?”
Visual reference
Quick reference
Routing Protocol Comparison
| Protocol | Metric | Max Hops | Algorithm | Type |
|---|---|---|---|---|
| RIP v2 | Hop count | 15 | Bellman-Ford | Distance vector |
| OSPF | Cost (bandwidth) | Unlimited | Dijkstra (SPF) | Link state |
| EIGRP | Composite metric | Unlimited | DUAL | Hybrid |
| IS-IS | Cost | Unlimited | Dijkstra | Link state |
| BGP | Policy / attributes | Unlimited | Path vector | Path vector |
RIP's 15-hop limit makes it unsuitable for large networks. OSPF and EIGRP dominate modern enterprise deployments.
Go deeper
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 200-301 practice question is part of Courseiva's free Cisco certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 200-301 exam.