CCNA IP Routing Practice Question
Exhibit
R1# interface GigabitEthernet0/0 ip address 10.1.12.1 255.255.255.0 ip ospf hello-interval 10 ip ospf dead-interval 40 ! router ospf 10 network 10.1.12.0 0.0.0.255 area 0 R2# interface GigabitEthernet0/0 ip address 10.1.12.2 255.255.255.0 ip ospf hello-interval 5 ip ospf dead-interval 20 ! router ospf 10 network 10.1.12.0 0.0.0.255 area 0
A network engineer enters the following configuration on R1 and R2, but R1 cannot form an OSPF adjacency with R2 on interface GigabitEthernet0/0.
R1# show running-config interface GigabitEthernet0/0 interface GigabitEthernet0/0 ip address 10.0.0.1 255.255.255.0 ip ospf hello-interval 10 ip ospf dead-interval 40 ip ospf 1 area 0
!
R2# show running-config interface GigabitEthernet0/0 interface GigabitEthernet0/0 ip address 10.0.0.2 255.255.255.0 ip ospf hello-interval 5 ip ospf dead-interval 20 ip ospf 1 area 0
What is the most likely cause of the failure?
⚠ Common exam trap
Remember that OSPF process IDs are locally significant and do not need to match between routers.
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 timers on the interface do not match.
The routers must agree on hello and dead timers to establish an OSPF adjacency. R1 uses hello=10 and dead=40, while R2 uses hello=5 and dead=20; this mismatch prevents the neighbor relationship. Although both routers share the same area (0) and subnet mask, and OSPF process IDs are locally significant (no match required), the timer difference is the root cause.
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 routers are in different OSPF areas.
Why it's wrong here
This would be a valid reason if one router were in area 0 and the other were in another area. In the exhibit, both routers place the same subnet into area 0, so the area setting is not the issue. The failure occurs at the interface parameter level instead.
When this WOULD be correct
In a different exam scenario, if the question specified that both routers were configured in the same OSPF area but were unable to establish an adjacency due to misconfiguration, then this option would be correct. For example, if both routers were in area 0 but had different area types, this could prevent adjacency formation.
- ✓
The OSPF timers on the interface do not match.
Why this is correct
This is correct because the interfaces are configured with different hello and dead intervals. OSPF neighbors expect those timers to align, and if they do not, the routers reject the neighbor relationship. The addressing and area assignment are fine, but the timer mismatch blocks adjacency formation.
- ✗
The subnet mask prevents multicast OSPF packets from being exchanged.
Why it's wrong here
The subnet mask shown is a normal /24 LAN mask and does not stop OSPF multicast traffic. OSPF commonly uses multicast on broadcast segments without any problem in that mask size. The real incompatibility is the timer mismatch, not the mask.
When this WOULD be correct
In a different scenario, if the question specified that the routers are on the same subnet but the subnet mask is incorrectly configured, preventing the multicast OSPF packets from being sent or received, then this option would be correct. For example, if R1 has a subnet mask of 255.255.255.0 and R2 has 255.255.0.0, they would not be able to communicate properly.
- ✗
The OSPF process IDs do not match on the two routers.
Why it's wrong here
This sounds believable because many learners assume OSPF process IDs must match between neighbors. In reality, the process ID is locally significant and is not exchanged as a neighbor-forming parameter. Two routers can still become neighbors with different local process numbers if the real link parameters match.
When this WOULD be correct
In a different scenario, if the question stated that R1 and R2 were configured to use different OSPF process IDs and were in the same area, this option would be correct, as OSPF requires matching process IDs for adjacency formation.
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 timers on the interface do not match.Correct answer▾
Why this is correct
This is correct because the interfaces are configured with different hello and dead intervals. OSPF neighbors expect those timers to align, and if they do not, the routers reject the neighbor relationship. The addressing and area assignment are fine, but the timer mismatch blocks adjacency formation.
✗The routers are in different OSPF areas.Wrong answer — click to see why▾
Why this is wrong here
This option is incorrect because OSPF can still form adjacencies between routers in different areas, provided they are correctly configured to do so. The issue in this scenario is related to OSPF timers, not area mismatches.
★ When this WOULD be the correct answer
In a different exam scenario, if the question specified that both routers were configured in the same OSPF area but were unable to establish an adjacency due to misconfiguration, then this option would be correct. For example, if both routers were in area 0 but had different area types, this could prevent adjacency formation.
Why candidates choose this
Candidates may choose this option due to a common misconception that OSPF adjacency can only be formed within the same area, leading them to overlook other potential issues like timer mismatches.
✗The subnet mask prevents multicast OSPF packets from being exchanged.Wrong answer — click to see why▾
Why this is wrong here
This option is incorrect because OSPF can still exchange multicast packets even if the subnet mask is not configured correctly, as long as the interfaces are up and configured for OSPF. The failure to form an adjacency is more likely due to mismatched OSPF timers.
★ When this WOULD be the correct answer
In a different scenario, if the question specified that the routers are on the same subnet but the subnet mask is incorrectly configured, preventing the multicast OSPF packets from being sent or received, then this option would be correct. For example, if R1 has a subnet mask of 255.255.255.0 and R2 has 255.255.0.0, they would not be able to communicate properly.
Why candidates choose this
Candidates might choose this option because they understand that OSPF relies on multicast communication, and they may incorrectly assume that any subnet mask issue would directly prevent OSPF adjacency, without considering the specific context of the question.
✗The OSPF process IDs do not match on the two routers.Wrong answer — click to see why▾
Why this is wrong here
This option is wrong because OSPF process IDs do not need to match for routers to form an adjacency; they only need to be in the same area and have matching network statements.
★ When this WOULD be the correct answer
In a different scenario, if the question stated that R1 and R2 were configured to use different OSPF process IDs and were in the same area, this option would be correct, as OSPF requires matching process IDs for adjacency formation.
Why candidates choose this
Candidates may choose this option due to a common misconception that OSPF process IDs are critical for adjacency, leading them to overlook other more relevant factors like area configuration or timer settings.
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
Learn chapter
OSPFv3 Single-Area Configuration for IPv6
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.
Key term
Subnet mask
A subnet mask is a 32-bit number that helps a computer or network device determine which part of an IP address identifies the network and which part identifies the host device on that 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.