CCNA IP Routing Practice Question
Which two statements about OSPF neighbor requirements on a shared Ethernet segment are correct? (Choose two.)
⚠ Common exam trap
A frequent exam trap is confusing the OSPF process ID with the area ID or router ID requirements. Candidates often think the process ID must match between neighbors, but it is locally significant and does not affect adjacency. Another common mistake is assuming router IDs must be identical; in fact, router IDs must be unique within the OSPF domain to prevent routing conflicts. Misunderstanding these distinctions can lead to selecting incorrect answers about neighbor requirements. The key is to focus on area ID and subnet matching for adjacency on shared Ethernet segments, not process ID or identical router IDs.
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
✓
They must be in the same OSPF area on that link.
Neighbors must agree on key parameters such as area ID and subnet, and they exchange Hello packets on the segment.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
They must be in the same OSPF area on that link.
Why this is correct
OSPF hello packets carry the area ID, and a router will only accept a neighbor if the area on the shared link matches its own. If the area IDs differ, the routers will drop each other's hello packets and never reach the 2-Way or Full state, so adjacency is impossible. Thus, being in the same area on that specific link is a strict prerequisite for forming a neighbor relationship.
- ✓
They must use the same subnet on the connected interfaces.
Why this is correct
OSPF on a broadcast or point-to-point network checks that the IP address and subnet mask of connected interfaces place both routers into the same subnet. If the subnet numbers or masks do not match, the interface is considered on a different link, and OSPF will not even insert the received hello packet into its neighbor table because the L3 reachability check fails. This ensures neighbors are directly reachable on the shared segment before any OSPF adjacency can form.
- ✗
They must have identical router IDs.
Why it's wrong here
Each OSPF router must have a unique Router ID (RID), typically the highest loopback or active interface IP address, because the RID is the primary key for OSPF neighbor and database descriptions. If two routers share the same RID, they will reject each other as duplicates, causing the neighbor relationship to flap or never establish. Identical RIDs are therefore invalid and harmful, not a requirement.
When this WOULD be correct
In a different question focused on OSPF configuration best practices, if the question asked about prerequisites for OSPF routing table synchronization or specific scenarios involving OSPF route selection, then having identical router IDs might be presented as a requirement to ensure consistent routing behavior.
- ✗
They must use the same process ID number on both routers.
Why it's wrong here
The OSPF process ID is a purely local parameter used to distinguish multiple OSPF processes running on the same router; it is not transmitted in any OSPF packet. Since hello, database descriptor, and link-state packets contain no process ID field, two routers can have completely different process IDs and still form a full adjacency. This makes the process ID irrelevant to neighbor requirements, unlike the area ID or subnet masking.
When this WOULD be correct
In a different exam scenario where the question asks about OSPF configuration requirements for routers that need to share routing information across multiple OSPF processes, having the same process ID would be necessary for them to exchange OSPF routing information effectively.
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.
✓They must be in the same OSPF area on that link.Correct answer▾
Why this is correct
OSPF hello packets carry the area ID, and a router will only accept a neighbor if the area on the shared link matches its own. If the area IDs differ, the routers will drop each other's hello packets and never reach the 2-Way or Full state, so adjacency is impossible. Thus, being in the same area on that specific link is a strict prerequisite for forming a neighbor relationship.
✗They must have identical router IDs.Wrong answer — click to see why▾
Why this is wrong here
Router IDs must be unique across the OSPF domain to avoid routing loops and confusion. Identical router IDs would cause OSPF to reject the neighbor relationship, as the router ID is used to identify each router uniquely in the LSDB.
★ When this WOULD be the correct answer
In a different question focused on OSPF configuration best practices, if the question asked about prerequisites for OSPF routing table synchronization or specific scenarios involving OSPF route selection, then having identical router IDs might be presented as a requirement to ensure consistent routing behavior.
Why candidates choose this
Students might confuse router ID with other parameters that need to match, such as area ID or subnet. They may think that identical router IDs are required for consistency, but in reality, uniqueness is critical.
✗They must use the same process ID number on both routers.Wrong answer — click to see why▾
Why this is wrong here
The OSPF process ID is locally significant and only used to identify the OSPF process on the local router. Neighbors can have different process IDs and still form an adjacency, as the process ID is not exchanged in OSPF packets.
★ When this WOULD be the correct answer
In a different exam scenario where the question asks about OSPF configuration requirements for routers that need to share routing information across multiple OSPF processes, having the same process ID would be necessary for them to exchange OSPF routing information effectively.
Why candidates choose this
Many students mistakenly believe that the process ID must match between neighbors, similar to how EIGRP requires the same autonomous system number. This is a common point of confusion between OSPF and other routing protocols.
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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OSPFv3 Single-Area Configuration for IPv6
Key term
OSPF
OSPF is a link-state routing protocol used to find the best path for data packets to travel across IP networks, like a smart GPS that recalculates routes when traffic changes.
Key term
OSPF
OSPF is a link-state routing protocol that uses the SPF algorithm to compute the shortest path to each destination within a single autonomous system.
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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.