Question 1hardmultiple choice
Full question →hardmultiple choiceObjective-mapped
Exhibit
R1 G0/0: 10.12.12.1/30, ip ospf 1 area 0 R2 G0/0: 10.12.12.2/30, ip ospf 1 area 1 Both interfaces are up/up.
Exhibit: R1 and R2 are directly connected. R1 does not show R2 as an OSPF neighbor. 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 area IDs do not match
An area mismatch prevents the neighbor relationship.
B
Distractor review
R1 must use a loopback as the router ID
A loopback helps stability but is not required to form adjacency.
C
Distractor review
The subnet mask is too small for OSPF to work
A /30 is perfectly valid on a point-to-point link.
D
Distractor review
R2 should be configured as the DR manually
DR election is not the issue here.
Common exam trap
Common exam trap: answer the scenario, not the keyword
A frequent exam trap is to blame the router ID configuration or subnet mask size for OSPF neighbor adjacency failure. Candidates might incorrectly believe that a loopback interface must be used as the router ID or that a /30 subnet mask is too small for OSPF to work. Another trap is thinking that manually configuring the Designated Router (DR) is necessary to form adjacency. In reality, OSPF adjacency depends primarily on matching area IDs. Misunderstanding this leads to incorrect troubleshooting and answer selection.
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 directly connected networks. These neighbors exchange routing information to build a consistent link-state database. A fundamental requirement for OSPF adjacency is that both routers must be in the same OSPF area, identified by the area ID. The area ID groups routers logically and controls the scope of link-state advertisements. When two routers are directly connected, they send OSPF hello packets to discover neighbors. These hello packets include critical parameters such as the area ID, hello and dead intervals, and router IDs. If the area IDs do not match, the routers will not recognize each other as valid neighbors, and adjacency will fail to form. This is because OSPF treats each area as a separate routing domain, and routers in different areas do not form neighbor relationships on the same link. A common exam trap is to assume that router ID configuration or subnet mask size causes adjacency failure. While router IDs are important for OSPF operation, they do not prevent neighbor formation if area IDs mismatch. Similarly, subnet masks like /30 are valid for point-to-point links and do not block adjacency. The Designated Router election also occurs after adjacency formation and does not prevent neighbor establishment. In practice, ensuring matching area IDs on connected interfaces is the first step to successful OSPF neighbor relationships.
KKey Concepts to Remember
- OSPF routers must be configured in the same area ID on directly connected interfaces to establish neighbor adjacency.
- OSPF forms neighbor relationships by exchanging hello packets that include area ID, router ID, and subnet information.
- An area ID mismatch prevents OSPF routers from recognizing each other as neighbors, even if interfaces are up and in the same subnet.
- OSPF router IDs can be manually set or automatically chosen, but using a loopback interface as router ID is not mandatory for adjacency.
- OSPF point-to-point links support subnet masks as small as /30 without impacting neighbor formation.
- The Designated Router (DR) election process does not affect the initial formation of OSPF neighbor relationships.
- OSPF adjacency requires matching hello and dead intervals, area IDs, and subnet connectivity to successfully form neighbor states.
- OSPF routers discard hello packets from neighbors with differing area IDs, preventing adjacency 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 routers must be configured in the same area ID on directly connected interfaces to establish neighbor adjacency.
What is the correct answer to this question?
The correct answer is: The area IDs do not match — OSPF neighbors on the same link must agree on the area ID. Here, one side is in area 0 and the other is in area 1, so adjacency never forms even though the interfaces are up and in the same subnet.
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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