Question 1hardmultiple choice
Full question →hardmultiple choiceObjective-mapped
Two OSPF routers connected over Ethernet fail to become neighbors. Their interfaces are up/up and in the same IPv4 subnet. One router uses area 0 and the other uses area 1 on the connecting interfaces. 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
Mismatched OSPF process IDs
OSPF process IDs are locally significant and do not need to match between neighbors.
B
Best answer
Mismatched OSPF areas on the shared link
Correct. The area mismatch is a neighbor-forming failure condition.
C
Distractor review
Missing default routes
Default routes are not required to establish adjacency.
D
Distractor review
Different router IDs
Different router IDs are expected; they must be unique, not equal.
Common exam trap
Common exam trap: answer the scenario, not the keyword
A common exam trap is assuming that OSPF process IDs must match between neighbors for adjacency to form. Many candidates mistakenly focus on process ID alignment, but OSPF process IDs are locally significant and do not need to match. The real cause of adjacency failure in this scenario is the mismatch in OSPF area IDs on the shared link. This subtle difference often leads to confusion, causing candidates to overlook the critical role of area consistency in neighbor formation and select incorrect answers related to process IDs or router IDs.
Technical deep dive
How to think about this question
Open Shortest Path First (OSPF) is a link-state routing protocol that establishes neighbor relationships between routers on a shared network segment to exchange routing information. For two OSPF routers to become neighbors, they must agree on several parameters, including the subnet, hello and dead intervals, authentication, and importantly, the OSPF area assigned to the interface. The area ID logically segments the OSPF domain, and routers in different areas do not form adjacencies directly on the same link. When two OSPF routers are connected over Ethernet and their interfaces are up with IP addresses in the same subnet, they still must be configured with the same OSPF area on that interface to form a neighbor adjacency. If one router’s interface is in area 0 and the other’s is in area 1, the routers will detect the mismatch during the OSPF hello packet exchange and will not progress beyond the Init or 2-Way state. This prevents the routers from exchanging link-state advertisements and building a consistent routing database. A common exam trap is confusing OSPF process IDs with areas. Process IDs are locally significant and do not need to match between neighbors, whereas area IDs must match on the shared link. In practical networks, mismatched areas on a shared segment cause adjacency failures and routing blackholes, so verifying area configuration consistency is critical for OSPF neighbor formation and stable routing.
KKey Concepts to Remember
- OSPF routers must be configured with the same area ID on a shared interface to successfully form neighbor adjacencies.
- OSPF process IDs are locally significant and do not affect neighbor relationships between routers.
- OSPF neighbors exchange hello packets that include area information to verify area consistency before adjacency formation.
- A mismatch in OSPF area IDs on the same subnet prevents routers from progressing beyond the initial neighbor discovery states.
- OSPF adjacency failure due to area mismatch stops link-state advertisement exchange and routing table updates.
- Different router IDs are required for OSPF neighbors to uniquely identify each router in the OSPF domain.
- Default routes are not required for OSPF neighbor formation; adjacency depends on interface and protocol parameters.
- Ethernet interfaces in the same IP subnet must share the same OSPF area to establish adjacency and exchange routing information.
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 be configured with the same area ID on a shared interface to successfully form neighbor adjacencies.
What is the correct answer to this question?
The correct answer is: Mismatched OSPF areas on the shared link — OSPF neighbors on the same link must agree on key parameters, including the area assigned to that interface. A mismatch prevents the adjacency from forming.
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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