Question 1hardmultiple choice
Full question →hardmultiple choiceObjective-mapped
Exhibit
R1# interface GigabitEthernet0/0 ip address 10.10.12.1 255.255.255.0 ip ospf hello-interval 10 ip ospf dead-interval 40 ! router ospf 1 network 10.10.12.0 0.0.0.255 area 0 R2# interface GigabitEthernet0/0 ip address 10.10.12.2 255.255.255.0 ip ospf hello-interval 5 ip ospf dead-interval 20 ! router ospf 1 network 10.10.12.0 0.0.0.255 area 0
A network engineer configures OSPF between R1 and R2, but the routers never become neighbors on GigabitEthernet0/0. Based on the exhibit, 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
The routers are using different OSPF areas.
This is wrong because both routers place the shared subnet into area 0.
B
Best answer
The OSPF interface timers do not match.
This is correct because OSPF neighbors on the same segment must agree on hello/dead intervals.
C
Distractor review
The routers must use the same hostname to become neighbors.
This is wrong because hostnames do not affect OSPF neighbor formation.
D
Distractor review
The subnet mask prevents multicast OSPF traffic.
This is wrong because a /24 mask does not prevent normal OSPF multicast behavior here.
Common exam trap
Common exam trap: answer the scenario, not the keyword
A frequent exam trap is assuming that OSPF adjacency forms as long as routers are in the same area and can ping each other. Candidates often overlook the necessity for matching hello and dead intervals, which are critical for neighbor discovery and maintenance. Because routers can still exchange ICMP packets, it may appear that the link is fully operational, leading to the mistaken belief that OSPF should work. This misunderstanding causes candidates to select incorrect answers related to area mismatches or subnet masks, ignoring the timer mismatch that actually prevents adjacency.
Technical deep dive
How to think about this question
OSPF (Open Shortest Path First) is a link-state routing protocol that establishes neighbor relationships between routers on a common network segment. These relationships are formed through the exchange of hello packets, which are multicast messages sent at regular intervals defined by the hello timer. The dead interval timer defines how long a router waits without receiving a hello before declaring the neighbor down. Both timers must match on connected interfaces for OSPF to form an adjacency. When two routers attempt to become OSPF neighbors, they first verify that they share the same area ID and subnet mask, ensuring they are in the same routing domain and network segment. Next, they compare hello and dead intervals; if these timers differ, the routers will not recognize each other as valid neighbors. This timer mismatch causes hello packets to be ignored, preventing the routers from progressing beyond the initial discovery phase. A common exam trap is assuming that IP connectivity or matching area IDs alone guarantee OSPF adjacency. In reality, OSPF requires strict parameter consistency, including timer values. Even if routers can ping each other, a hello/dead interval mismatch will block neighbor formation. This subtlety often confuses candidates because basic IP reachability tests succeed, masking the underlying protocol disagreement.
KKey Concepts to Remember
- OSPF routers must match hello and dead interval timers on their interfaces to successfully form neighbor adjacencies.
- OSPF hello packets are multicast messages used to discover and maintain neighbor relationships on a shared network segment.
- OSPF area IDs must be consistent between routers on the same link to ensure proper adjacency formation and routing information exchange.
- OSPF neighbor relationships do not depend on router hostnames; hostnames are local identifiers and do not affect protocol operation.
- OSPF uses multicast addresses 224.0.0.5 and 224.0.0.6 to send hello and database description packets, which require proper subnet masks to function.
- Mismatch in OSPF interface timers causes routers to reject each other's hello packets, preventing adjacency despite IP connectivity.
- OSPF adjacency formation requires agreement on several parameters including area ID, hello/dead intervals, authentication, and subnet mask.
- OSPF routers can ping each other but still fail to form neighbor relationships if key protocol parameters like timers do not match.
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 match hello and dead interval timers on their interfaces to successfully form neighbor adjacencies.
What is the correct answer to this question?
The correct answer is: The OSPF interface timers do not match. — The most likely cause is a hello/dead timer mismatch. In practical terms, both routers are on the same subnet and both are trying to run OSPF in area 0, so the obvious items look correct. But OSPF does not form adjacency based on IP reachability alone. It also checks whether key interface parameters agree. Hello and dead intervals are among those required parameters. On R1, the hello interval is 10 seconds and the dead interval is 40 seconds. On R2, the hello interval is 5 seconds and the dead interval is 20 seconds. That mismatch is enough to prevent the neighbor relationship from forming. This is a common CCNA troubleshooting pattern because the interfaces can still ping each other, which makes the failure look less obvious at first glance.
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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