The answer is an OSPF hello and dead timer mismatch. This is the most likely cause of the adjacency failure because OSPF requires that neighboring routers agree on these interface timers to form a neighbor relationship; if the hello or dead intervals differ, the routers will not recognize each other’s Hello packets as valid, leaving the state stuck in INIT or EXSTART. On the CCNA 200-301 v2 exam, this concept tests your understanding of OSPF neighbor establishment requirements, often appearing as a configuration verification or troubleshooting scenario where timers are manually set on one side but left at defaults on the other—a common trap is assuming OSPF process IDs must match, but they are locally significant and do not affect adjacency. A quick memory tip: think of the hello and dead timers as a handshake rhythm—if one router waves every 10 seconds and expects a reply within 40, while the other waves every 5 seconds and expects a reply within 20, they will never sync up.
CCNA IP Routing Practice Question
This 200-301 practice question tests your understanding of ip routing. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. A key principle to apply: oSPF routers must have matching hello and dead interval timers on interfaces to successfully form neighbor adjacencies.. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
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?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
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
The routers are in different OSPF areas.
Why wrong: 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.
B
The OSPF timers on the interface do not match.
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.
C
The subnet mask prevents multicast OSPF packets from being exchanged.
Why wrong: 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.
D
The OSPF process IDs do not match on the two routers.
Why wrong: 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.
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.
Key principle: OSPF routers must have matching hello and dead interval timers on interfaces to successfully form neighbor adjacencies.
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.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
OSPF routers must have matching hello and dead interval timers on interfaces to successfully form neighbor adjacencies.
✗
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?”
Common exam traps
Common exam trap: answer the scenario, not the keyword
Remember that OSPF process IDs are locally significant and do not need to match between routers.
Trap categories for this question
Command / output trap
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.
Detailed technical explanation
How to think about this question
Open Shortest Path First (OSPF) is a link-state routing protocol that requires routers to form neighbor adjacencies before exchanging routing information. For two routers to become OSPF neighbors, several interface parameters must match exactly, including the hello interval and dead interval timers. The hello interval defines how often a router sends hello packets to discover and maintain neighbor relationships, while the dead interval specifies the time a router waits without receiving a hello before declaring the neighbor down.
OSPF adjacency formation follows strict rules: routers must be in the same subnet, configured with the same area ID, use compatible authentication settings, and have matching network types and timers on the interfaces involved. The hello and dead timers are critical because mismatched timers cause routers to perceive each other as incompatible, preventing adjacency formation. The OSPF process ID is locally significant and does not affect neighbor relationships.
A common exam trap is assuming that OSPF process IDs must match between neighbors or that subnet masks alone can block adjacency. In practice, mismatched hello and dead intervals are a frequent cause of adjacency failure. Cisco routers will not form OSPF adjacencies if these timers differ, as the routers cannot synchronize their neighbor state machines. Understanding this behavior is essential for troubleshooting OSPF neighbor issues and ensuring stable routing in Cisco networks.
KKey Concepts to Remember
OSPF routers must have matching hello and dead interval timers on interfaces to successfully form neighbor adjacencies.
The OSPF process ID is locally significant and does not need to match between routers for adjacency formation.
OSPF adjacency requires routers to be in the same subnet and configured with the same area ID on the participating interfaces.
Mismatched OSPF hello and dead timers cause routers to reject neighbor relationships, preventing adjacency formation.
OSPF uses multicast hello packets sent at the hello interval to discover and maintain neighbor relationships.
The dead interval timer defines how long a router waits without receiving a hello before declaring a neighbor down.
Subnet masks do not block OSPF multicast packets if the routers are in the same subnet and area.
OSPF adjacency formation depends on matching interface parameters including area, authentication, network type, and timers.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
OSPF routers must have matching hello and dead interval timers on interfaces to successfully form neighbor adjacencies.
Real-world example
How this comes up in practice
A network engineer at a university connects two campus buildings via a fibre link. Both routers run OSPF, but no adjacency forms — even though both routers can ping each other. The engineer finds one router is in area 0 and the other in area 1. OSPF adjacency requires matching area numbers, hello/dead timers, and network type. IP reachability alone is not enough.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 200-301 question in full detail.
Review oSPF routers must have matching hello and dead interval timers on interfaces to successfully form neighbor adjacencies., then practise related 200-301 questions on the same topic to reinforce the concept.
IP Routing — This question tests IP Routing — OSPF routers must have matching hello and dead interval timers on interfaces to successfully form neighbor adjacencies..
What is the correct answer to this question?
The correct answer is: 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.
What should I do if I get this 200-301 question wrong?
Review oSPF routers must have matching hello and dead interval timers on interfaces to successfully form neighbor adjacencies., then practise related 200-301 questions on the same topic to reinforce the concept.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
OSPF routers must have matching hello and dead interval timers on interfaces to successfully form neighbor adjacencies.
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