Question 1hardmultiple choice
Full question →hardmultiple choiceObjective-mapped
Exhibit
R1 G0/0: ip 10.1.12.1/30, ip ospf hello-interval 10, ip ospf dead-interval 40 R2 G0/0: ip 10.1.12.2/30, ip ospf hello-interval 5, ip ospf dead-interval 20 Both interfaces are up/up and in area 0.
Exhibit: R1 cannot form an OSPF adjacency with R2 on G0/0. 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
Distractor review
MTU mismatch
Distractor.
B
Best answer
Hello and dead timer mismatch
Correct choice.
C
Distractor review
Missing router ID on both routers
Distractor.
D
Distractor review
A duplicate static route to 10.1.12.0/30
Distractor.
Common exam trap
Common exam trap: answer the scenario, not the keyword
A common exam trap is selecting MTU mismatch or missing router IDs as the cause of OSPF adjacency failure. While MTU mismatches can cause adjacency issues during database exchange, they do not prevent the initial neighbor relationship from forming. Missing router IDs do not block adjacency because routers automatically generate IDs if none are configured. Another trap is confusing static routes with neighbor discovery; static routes do not influence OSPF adjacency. The key is to focus on timer mismatches, as hello and dead intervals must be identical for routers to recognize each other as neighbors and establish adjacency.
Technical deep dive
How to think about this question
OSPF (Open Shortest Path First) is a link-state routing protocol that relies on neighbor adjacencies to exchange routing information. Routers send hello packets at regular intervals to discover neighbors and maintain adjacency. The hello interval defines how often these packets are sent, while the dead interval specifies how long a router waits without receiving a hello before declaring the neighbor down. Both timers must match on connected interfaces for adjacency to form. When two OSPF routers connect on a broadcast or point-to-point network, they compare several parameters during the neighbor discovery process, including hello and dead timers, area ID, subnet, and authentication settings. If any of these parameters differ, the routers will not establish an adjacency. The hello and dead timer mismatch is a common cause because these timers control neighbor liveness detection. If one router expects hellos every 10 seconds but the other sends them every 30 seconds, the adjacency will fail. Exam candidates often mistake other issues such as MTU mismatch or missing router IDs as causes for adjacency failure. While MTU mismatches can delay adjacency in some cases, they do not typically prevent it outright. Router IDs are automatically assigned if not configured, so missing IDs do not block adjacency. Static routes are unrelated to neighbor formation. Understanding that hello and dead timer consistency is mandatory helps avoid these traps and ensures reliable OSPF neighbor relationships in practical networks.
KKey Concepts to Remember
- OSPF routers must have matching hello and dead timers on interfaces to successfully form neighbor adjacencies.
- OSPF hello packets are sent periodically to discover and maintain neighbor relationships on broadcast and point-to-point links.
- A mismatch in OSPF hello or dead intervals causes routers to reject each other as neighbors, preventing adjacency formation.
- OSPF adjacency formation requires interfaces to be in the same subnet and configured for the same OSPF area.
- MTU mismatches can prevent OSPF adjacency in some cases, but hello and dead timer mismatches are more common causes.
- Missing router IDs do not prevent OSPF adjacency; routers automatically generate router IDs if none are configured.
- Static routes do not affect OSPF adjacency formation because OSPF neighbor discovery is independent of routing table entries.
- OSPF adjacency parameters such as hello and dead timers must be consistent to ensure stable and reliable routing updates.
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 timers on interfaces to successfully form neighbor adjacencies.
What is the correct answer to this question?
The correct answer is: Hello and dead timer mismatch — The interfaces are in the same subnet and both routers are configured for OSPF area 0, but the hello timers do not match. OSPF neighbors on broadcast and point-to-point links must agree on key parameters such as hello and dead timers before an adjacency can form.
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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