CCNA IP Routing Practice Question
Exhibit
R1# interface GigabitEthernet0/0 ip address 10.0.12.1 255.255.255.252 ip ospf authentication message-digest ip ospf message-digest-key 1 md5 cisco123 ! router ospf 1 network 10.0.12.0 0.0.0.3 area 0 R2# interface GigabitEthernet0/0 ip address 10.0.12.2 255.255.255.252 ip ospf authentication message-digest ip ospf message-digest-key 1 md5 cisco321 ! router ospf 1 network 10.0.12.0 0.0.0.3 area 0
R1 and R2 are directly connected and running OSPF. They can ping each other, the area matches, and the timers match, but they still do not become neighbors. What is the most likely cause?
⚠ Common exam trap
A frequent exam trap is to overlook OSPF authentication mismatches when routers have IP connectivity and matching area IDs. Candidates may incorrectly assume that because the routers can ping each other and timers match, adjacency must form. However, if OSPF authentication keys differ, routers silently reject OSPF packets, preventing neighbor formation. This trap exploits the misconception that IP reachability alone ensures OSPF adjacency, ignoring the critical role of matching authentication parameters in the OSPF neighbor negotiation process.
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 keys do not match.
The most likely cause is a mismatch in OSPF authentication keys. Even though the routers have IP connectivity, matching area IDs, and identical timers, OSPF adjacency requires that authentication parameters also match. If authentication is enabled on both sides but the keys differ, OSPF packets are silently rejected, preventing neighbor formation.
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 keys do not match.
Why this is correct
This is the correct cause. In OSPF, if MD5 authentication is enabled but the configured authentication keys (or key IDs) do not match between the two routers, all OSPF hello packets are silently dropped. The routers remain reachable at Layer 3 via ping because IP forwarding does not depend on OSPF authentication, but they will never form a neighbor adjacency or exchange routing information.
- ✗
The subnet mask is too small for OSPF to operate.
Why it's wrong here
This is incorrect because OSPF functions perfectly on a /30 subnet, which is the standard prefix length for point-to-point router links. A /30 mask provides two usable host addresses, exactly enough for the two router interfaces, and OSPF does not require a larger subnet or more host bits. A subnet mask that is 'too small' would be one that has no valid host addresses, such as /31 or /32, but even /31 can be used for OSPF point-to-point links in many implementations. Thus, the /30 mask is not a problem here.
When this WOULD be correct
In a different scenario where the question states that R1 and R2 are configured with OSPF but are on different subnets due to a misconfigured subnet mask, leading to a lack of reachability, this option would be correct. For example, if R1 is on 192.168.1.0/30 and R2 is on 192.168.1.4/30, they cannot form an OSPF neighbor relationship.
- ✗
The routers must use different process IDs.
Why it's wrong here
This is wrong because OSPF process IDs are locally significant only; they identify the OSPF process on a single router and do not need to match between neighbors. In fact, two routers can form a full OSPF adjacency even if one uses process ID 1 and the other uses process ID 100, as long as the area ID, network type, subnet, and authentication parameters (if any) are compatible. The requirement that they differ is a common misconception; the routers may use the same or different process IDs without affecting neighbor formation.
When this WOULD be correct
In a different scenario, if the question stated that R1 and R2 are in the same area but have different OSPF process IDs, then this option would be correct, as OSPF routers must have the same process ID to form a neighbor relationship.
- ✗
The interfaces must be configured as switch trunks.
Why it's wrong here
Trunk interfaces are used to carry multiple VLANs over a single link, typically between switches. OSPF neighbour formation over a direct router-to-router Ethernet link requires the interface to be in the same subnet with matching OSPF parameters, not a trunk configuration. This option is tempting because trunking is a common Layer 2 requirement for inter-switch links, but the scenario involves routers, not switches, so trunking is irrelevant to OSPF adjacency.
When this WOULD be correct
In a different scenario where the question specifies that R1 and R2 are connected via a switch and need to establish OSPF over multiple VLANs, the interfaces would need to be configured as switch trunks to allow OSPF traffic across those VLANs. If the trunking is misconfigured, OSPF neighbors would not form.
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 keys do not match.Correct answer▾
Why this is correct
This is the correct cause. In OSPF, if MD5 authentication is enabled but the configured authentication keys (or key IDs) do not match between the two routers, all OSPF hello packets are silently dropped. The routers remain reachable at Layer 3 via ping because IP forwarding does not depend on OSPF authentication, but they will never form a neighbor adjacency or exchange routing information.
✗The subnet mask is too small for OSPF to operate.Wrong answer — click to see why▾
Why this is wrong here
OSPF does not require a specific subnet mask size; it works on any subnet, including /30 links. The issue described (ping working but OSPF not forming adjacency) is not caused by a subnet mask being too small.
★ When this WOULD be the correct answer
In a different scenario where the question states that R1 and R2 are configured with OSPF but are on different subnets due to a misconfigured subnet mask, leading to a lack of reachability, this option would be correct. For example, if R1 is on 192.168.1.0/30 and R2 is on 192.168.1.4/30, they cannot form an OSPF neighbor relationship.
Why candidates choose this
Students may confuse OSPF's requirement for matching subnet masks on the same link (which is true for OSPF network type broadcast) with the mask being too small. However, a /30 mask is perfectly valid and commonly used for point-to-point links.
✗The routers must use different process IDs.Wrong answer — click to see why▾
Why this is wrong here
OSPF process IDs are locally significant and do not need to match between routers. Two routers can have different process IDs and still form a neighbor adjacency as long as other parameters (area, timers, authentication) match.
★ When this WOULD be the correct answer
In a different scenario, if the question stated that R1 and R2 are in the same area but have different OSPF process IDs, then this option would be correct, as OSPF routers must have the same process ID to form a neighbor relationship.
Why candidates choose this
Students often confuse OSPF process IDs with EIGRP autonomous system numbers, which must match. This leads to the mistaken belief that OSPF process IDs must also match.
✗The interfaces must be configured as switch trunks.Wrong answer — click to see why▾
Why this is wrong here
OSPF runs on routed interfaces (Layer 3), not on switch trunks (Layer 2). Configuring interfaces as switch trunks would prevent OSPF from operating because trunks are used for VLAN tagging, not for routing protocols.
★ When this WOULD be the correct answer
In a different scenario where the question specifies that R1 and R2 are connected via a switch and need to establish OSPF over multiple VLANs, the interfaces would need to be configured as switch trunks to allow OSPF traffic across those VLANs. If the trunking is misconfigured, OSPF neighbors would not form.
Why candidates choose this
Some students may think that OSPF requires trunking because they have seen OSPF configured on VLAN interfaces or subinterfaces, but that is different from configuring a physical interface as a trunk port.
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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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.
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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.