Question 1hardmultiple choice
Full question →hardmultiple choiceObjective-mapped
Two routers are in the same OSPF area and on the same subnet, but they do not form an adjacency. One interface uses a hello interval of 10 seconds and the other uses 5 seconds. What is the most likely cause?
Answer choices
Why each option matters
Good practice is not just finding the correct option. The wrong answers often show the exact trap the exam wants you to fall into.
A
Best answer
The OSPF hello timers do not match.
This is correct because OSPF neighbors must agree on hello timing in order to form an adjacency.
B
Distractor review
The routers need matching hostnames.
This is wrong because hostnames do not determine OSPF adjacency formation.
C
Distractor review
The subnet must be changed to /24 before OSPF can work.
This is wrong because OSPF can operate on many subnet sizes; the issue described is timer mismatch.
D
Distractor review
The routers must both use static routes first.
This is wrong because static routes are not required for OSPF adjacency formation.
Common exam trap
Common exam trap: answer the scenario, not the keyword
A common exam trap is assuming that because two routers can ping each other on the same subnet, their OSPF adjacency will form automatically. The trap lies in overlooking the importance of matching OSPF hello and dead timers. Many candidates focus only on IP addressing or area configuration and miss that OSPF requires exact timer agreement. This leads to confusion when adjacency never forms despite correct IP setup. The exam tests your understanding that OSPF is a protocol with strict neighbor parameter requirements, not just IP reachability.
Technical deep dive
How to think about this question
OSPF (Open Shortest Path First) is a link-state routing protocol that relies on periodic hello packets to discover and maintain neighbor relationships on a shared network segment. Each router interface configured for OSPF sends hello packets at a configured hello interval, and neighbors must agree on this interval to recognize each other as valid peers. The hello interval controls how often these packets are sent, and the dead interval defines how long a router waits without receiving a hello before declaring the neighbor down. For two OSPF routers on the same subnet and area to form an adjacency, they must match several parameters exactly, including hello and dead intervals, area ID, subnet mask, and authentication if configured. If the hello intervals differ, one router will send hellos at a different rate than the other expects, causing the adjacency process to fail. This mismatch prevents the routers from establishing a stable neighbor relationship, even though basic IP connectivity like ping may still work. This scenario is a classic exam trap because it is easy to assume that IP reachability guarantees OSPF adjacency. However, OSPF requires strict parameter matching beyond just IP addressing. The practical impact is that network engineers must verify and align OSPF timers on all routers in the same area and subnet to ensure proper adjacency formation and routing table synchronization. Ignoring timer mismatches can lead to persistent routing issues despite apparent network connectivity.
KKey Concepts to Remember
- OSPF routers must have matching hello and dead intervals on interfaces within the same area and subnet to successfully form neighbor adjacencies.
- OSPF hello packets are sent periodically to discover and maintain neighbor relationships, and mismatched timers cause adjacency failures despite IP reachability.
- OSPF adjacency formation requires agreement on several parameters including hello interval, dead interval, area ID, subnet mask, and authentication settings.
- OSPF uses hello intervals to detect neighbor liveness, and if one router’s hello interval differs, it will not recognize the other as a valid neighbor.
- OSPF adjacency failures due to timer mismatches are common troubleshooting issues because interfaces may still ping but never fully synchronize routing information.
- Hostname matching is not required for OSPF adjacency; OSPF neighbors identify each other by router IDs and interface parameters, not device names.
- OSPF does not require static routes to form adjacencies; it dynamically discovers neighbors using hello packets and forms adjacencies based on protocol parameters.
- OSPF supports multiple subnet masks and does not require a /24 subnet; adjacency depends on matching network parameters, not specific subnet sizes.
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.
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More questions from this exam
Keep practising from the same exam bank, or move into a focused topic page if this question exposed a weak area.
Question 1
A router learns the same prefix from both OSPF and EIGRP. Which route is installed by default?
Question 2
A router shows this output: R1#show ip ospf neighbor Neighbor ID Pri State Dead Time Address Interface 10.1.1.2 1 FULL/DR 00:00:34 192.168.12.2 GigabitEthernet0/0 10.1.1.3 1 2WAY/DROTHER 00:00:39 192.168.12.3 GigabitEthernet0/0 Which statement is correct?
Question 3
What is the OSPF metric called?
Question 4
A non-root switch has two uplinks toward the root bridge. One path has a lower total STP cost than the other. What role will the lower-cost uplink have?
Question 5
A router interface applies this ACL inbound: 10 deny tcp any any eq 80 20 permit ip any any A user reports that web browsing to a server by IP address fails, but ping works. Which statement best explains the behavior?
Question 6
A router learns route 198.51.100.0/24 from OSPF with AD 110 and also has a static route to the same prefix configured with AD 150. Which route is installed?
FAQ
Questions learners often ask
What does this 200-301 question test?
OSPF routers must have matching hello and dead intervals on interfaces within the same area and subnet to successfully form neighbor adjacencies.
What is the correct answer to this question?
The correct answer is: The OSPF hello timers do not match. — An OSPF timer mismatch is the most likely cause. In plain language, the routers are trying to discover each other, but they are speaking on different timing expectations. OSPF neighbors on the same segment must agree on certain parameters before they trust each other enough to form an adjacency, and the hello and dead timers are among those critical values. This is a very common OSPF troubleshooting issue because the interfaces may still be reachable with ping and may even appear correctly addressed and placed in the same area. But OSPF is stricter than simple IP reachability. If the hello interval does not match, the adjacency usually fails before it becomes fully operational.
What should I do if I get this 200-301 question wrong?
Then try more questions from the same exam bank and focus on understanding why the wrong options are tempting.
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