Question 1hardmultiple choice
Full question →hardmultiple choiceObjective-mapped
Two routers are directly connected and running OSPF. Their IP addresses and hello timers match, but they still do not become neighbors. One side is configured for area 0 and the other for area 1 on the shared link. 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 interfaces are in different OSPF areas on the shared link.
This is correct because OSPF neighbors on the same segment must agree on the area.
B
Distractor review
The routers need identical hostnames.
This is wrong because hostnames do not determine OSPF adjacency.
C
Distractor review
The link must be converted to a trunk.
This is wrong because OSPF adjacency on routed interfaces does not require trunking.
D
Distractor review
The routers must 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 frequent exam trap is assuming that matching IP addresses and hello timers alone guarantee OSPF adjacency. Candidates often overlook the OSPF area configuration, which must be identical on both sides of a shared link. This mistake leads to confusion because the routers appear connected and can exchange packets, but OSPF adjacency never forms. The trap is focusing on interface parameters like timers or IP addressing while ignoring the fundamental requirement that both interfaces must belong to the same OSPF area to establish neighbor relationships.
Technical deep dive
How to think about this question
OSPF (Open Shortest Path First) is a link-state routing protocol that organizes routers into areas to optimize routing and reduce overhead. Each OSPF interface must be assigned to a specific area, and routers form neighbor adjacencies only if they share the same area on the connected segment. This area concept segments the network logically, allowing routers to exchange link-state advertisements (LSAs) only within their area, which improves scalability and stability. When two routers are directly connected and running OSPF, they exchange hello packets to establish adjacency. For adjacency to form successfully, several parameters must match, including hello and dead intervals, subnet mask, authentication, and crucially, the OSPF area ID on the interface. If one router’s interface is configured in area 0 and the other in area 1 on the same link, they will not recognize each other as neighbors because they belong to different OSPF areas. This mismatch prevents the exchange of LSAs and the formation of a neighbor relationship. A common exam trap is to overlook the importance of area consistency on shared links and focus only on IP addressing or timers. Physically connected routers with matching hello timers but mismatched areas will fail to form adjacency, leading to routing issues. In practical networks, ensuring area consistency on all interfaces participating in OSPF is critical to maintain proper routing topology and avoid silent failures in neighbor formation.
KKey Concepts to Remember
- OSPF requires that all routers on a shared link must be configured with the same area ID to form neighbor adjacencies.
- OSPF adjacency formation depends on matching hello and dead intervals, subnet masks, authentication, and area IDs on connected interfaces.
- Routers in different OSPF areas do not establish neighbor relationships even if IP addresses and timers match.
- OSPF areas segment the network logically to reduce routing overhead and control the scope of link-state advertisements.
- OSPF interfaces assigned to different areas on the same physical link cause adjacency failure and prevent routing updates.
- OSPF adjacency failure due to area mismatch can appear as a connectivity issue despite correct IP addressing and timer settings.
- The 'show ip ospf interface' command helps verify area configuration and troubleshoot adjacency problems effectively.
- OSPF adjacency rules enforce strict parameter matching to ensure consistent and reliable routing topology formation.
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 requires that all routers on a shared link must be configured with the same area ID to form neighbor adjacencies.
What is the correct answer to this question?
The correct answer is: The interfaces are in different OSPF areas on the shared link. — An OSPF area mismatch is the most likely cause. In plain language, the routers may be physically connected and able to exchange packets, but OSPF still requires that both sides agree on the area associated with the shared segment. If one interface belongs to area 0 and the other belongs to area 1, the routers do not view the segment in the same OSPF context and the adjacency fails. This is a classic CCNA troubleshooting case because the addressing can look perfect while the protocol still refuses to form a neighbor relationship. OSPF is strict about several interface-level values, and the area assignment is one of the most important.
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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