CCNA IP Routing Practice Question
Exhibit
R1 g0/0: area 0, ip ospf authentication message-digest R2 g0/0: area 1, no authentication
Exhibit: An OSPFv2 adjacency between two routers on Ethernet is not forming. Which two mismatches would directly prevent the routers from becoming neighbors?
⚠ Common exam trap
A common exam trap is selecting options like different hostnames or loopback addresses as causes for OSPF adjacency failure. Candidates may mistakenly believe that router identification parameters affect neighbor formation. However, OSPF adjacency strictly depends on protocol parameters such as area ID and authentication. Hostnames and loopback interfaces are used for management or router ID purposes but do not block adjacency. Misunderstanding this can lead to incorrect troubleshooting steps or exam answers, wasting valuable time and causing confusion.
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
✓
Different OSPF area assignments on the interfaces
On an OSPFv2 network, area mismatch and authentication mismatch both directly prevent adjacency formation. Mismatched timers (such as hello or dead intervals) also block adjacency on all network types, including Ethernet. In this scenario, the correct choices are area mismatch (A) and authentication mismatch (B).
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Different OSPF area assignments on the interfaces
Why this is correct
When two routers on the same Ethernet segment have different OSPF area assignments, the area ID embedded in each Hello packet differs. A router receiving a Hello with a mismatched area ID discards the packet, preventing the neighbor relationship from even reaching the 2-Way state. Thus, area numbers must match exactly on the shared link for OSPFv2 adjacency to form.
- ✓
Authentication mismatch between the interfaces
Why this is correct
An authentication mismatch occurs when one OSPF interface is configured with authentication and the other is not, or when the keys/credentials differ. Since OSPFv2 embeds authentication data in every OSPF packet, a router that receives a Hello with an invalid or missing authentication code simply drops it. This silent failure prevents any adjacency negotiation from starting, keeping the neighbors down.
- ✗
Different hostnames on the routers
Why it's wrong here
Hostnames are purely local references used for CLI display and managing the device; they are not transmitted in any OSPF packet. OSPF adjacency depends only on protocol parameters such as area ID, timers, network type, and authentication. Therefore, different hostnames on the routers have absolutely no effect on the OSPF adjacency formation process.
When this WOULD be correct
In a question about EIGRP neighbor formation over Frame Relay, mismatched hostnames could prevent adjacency if the hostname is used in the EIGRP authentication process (e.g., MD5 authentication with key-chain).
- ✗
Different loopback addresses used for management
Why it's wrong here
Loopback addresses, even if used for management or as the OSPF router ID, do not directly participate in the OSPF Hello exchange on an Ethernet link. While a different router ID might slightly alter tie-breaking in DR/BDR elections, it never prevents adjacency as long as the interface parameters match. The management loopbacks are not on the same physical subnet as the adjacency interface, so they are irrelevant to the OSPF neighbor state machine.
When this WOULD be correct
In a question about OSPF router ID selection, if two routers have the same router ID (e.g., due to identical loopback addresses), they would not form an adjacency. The correct answer would be that identical router IDs prevent neighbor formation.
- ✗
One router using SSH version 2
Why it's wrong here
SSH version 2 is a transport layer protocol for securely accessing a network device's CLI; it is completely unrelated to OSPF. OSPFv2 runs directly over IP protocol number 89 and has its own message format, independent of any application protocols. Thus, whether a router supports SSH version 1 or version 2 has zero impact on its ability to form an OSPFv2 adjacency.
When this WOULD be correct
In a question about why SSH access to a router fails between two devices, mismatched SSH versions (e.g., one using SSHv1 and the other SSHv2) would prevent secure connection.
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.
✓Different OSPF area assignments on the interfacesCorrect answer▾
Why this is correct
When two routers on the same Ethernet segment have different OSPF area assignments, the area ID embedded in each Hello packet differs. A router receiving a Hello with a mismatched area ID discards the packet, preventing the neighbor relationship from even reaching the 2-Way state. Thus, area numbers must match exactly on the shared link for OSPFv2 adjacency to form.
✗Different hostnames on the routersWrong answer — click to see why▾
Why this is wrong here
OSPF neighbor formation does not depend on hostnames; hostnames are only used for router identification in the routing table and do not affect OSPF adjacency.
★ When this WOULD be the correct answer
In a question about EIGRP neighbor formation over Frame Relay, mismatched hostnames could prevent adjacency if the hostname is used in the EIGRP authentication process (e.g., MD5 authentication with key-chain).
Why candidates choose this
Candidates may confuse hostname with router ID or think that OSPF uses hostnames for neighbor verification, leading them to incorrectly select this option.
✗Different loopback addresses used for managementWrong answer — click to see why▾
Why this is wrong here
Different loopback addresses do not affect OSPF neighbor formation; OSPF uses the router ID, which can be set manually or derived from the highest loopback IP, but mismatched loopback addresses themselves do not prevent adjacency.
★ When this WOULD be the correct answer
In a question about OSPF router ID selection, if two routers have the same router ID (e.g., due to identical loopback addresses), they would not form an adjacency. The correct answer would be that identical router IDs prevent neighbor formation.
Why candidates choose this
Candidates may confuse loopback addresses with OSPF router IDs, thinking mismatched loopbacks cause issues, or they may overgeneralize that any IP mismatch prevents OSPF adjacency.
✗One router using SSH version 2Wrong answer — click to see why▾
Why this is wrong here
OSPFv2 neighbor formation does not depend on SSH version; SSH is a management protocol for secure remote access, unrelated to OSPF adjacency requirements.
★ When this WOULD be the correct answer
In a question about why SSH access to a router fails between two devices, mismatched SSH versions (e.g., one using SSHv1 and the other SSHv2) would prevent secure connection.
Why candidates choose this
Candidates may confuse security protocols used for device management with routing protocol authentication, thinking any mismatch in security settings blocks OSPF adjacency.
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
Related to this question
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OSPFv3 Single-Area Configuration for IPv6
Key term
Ethernet
Ethernet is a wired networking technology that connects devices like computers, printers, and servers to each other and to the internet using cables.
Key term
OSPFv2
OSPFv2 is a network routing protocol that helps routers share information about the best paths to send data across an IP network.
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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.