Question 1hardmultiple choice
Full question →hardmultiple choiceObjective-mapped
Exhibit
R1# show ip ospf interface g0/0 GigabitEthernet0/0 is up, line protocol is up Internet Address 10.1.12.1/30, Area 0 MTU 1500 bytes R2# show ip ospf interface g0/0 GigabitEthernet0/0 is up, line protocol is up Internet Address 10.1.12.2/30, Area 0 MTU 1400 bytes
R1 and R2 are directly connected and running OSPF. The IP addressing is correct and both routers are in area 0, but they do not form an adjacency. 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
Best answer
The OSPF MTU values do not match on the connected interfaces.
This is correct because MTU mismatch is a known cause of OSPF adjacency problems during database exchange.
B
Distractor review
The routers must use identical hostnames before OSPF can form neighbors.
This is wrong because hostnames do not determine OSPF adjacency.
C
Distractor review
The routers must use VLAN 1 for OSPF to operate.
This is wrong because OSPF is not dependent on VLAN 1 in this routed-interface scenario.
D
Distractor review
The interfaces need to be configured as trunks instead of routed ports.
This is wrong because OSPF commonly runs over routed interfaces, not switch trunks.
Common exam trap
Common exam trap: answer the scenario, not the keyword
A common exam trap is assuming that because routers can ping each other and share the same OSPF area, adjacency must form correctly. This overlooks the critical role of MTU matching. Candidates may mistakenly believe hostname matching or VLAN configurations affect OSPF adjacency, but these do not influence OSPF neighbor formation. The subtlety of MTU mismatch causing adjacency failure during database exchange is often missed, leading to incorrect troubleshooting steps or answer choices. Recognizing that OSPF adjacency depends on interface parameters beyond basic IP connectivity is key to avoiding this trap.
Technical deep dive
How to think about this question
OSPF (Open Shortest Path First) is a link-state routing protocol that establishes neighbor adjacencies to exchange routing information within the same area. For two routers to form an OSPF adjacency, several parameters must match, including area ID, hello and dead intervals, authentication settings, and importantly, the MTU (Maximum Transmission Unit) size on the interfaces. The MTU defines the largest packet size that can be transmitted without fragmentation. If the MTU values differ, OSPF adjacency formation can stall during the database exchange phase because routers reject packets that exceed their MTU size. During OSPF neighbor establishment, routers first exchange hello packets to discover neighbors and verify basic parameters. Once neighbors are recognized, they proceed to exchange link-state databases. If the MTU sizes do not match, the routers detect this mismatch and prevent the adjacency from fully forming to avoid packet loss or fragmentation issues. This behavior is specific to OSPF and is a common troubleshooting point when IP connectivity and area configurations appear correct but adjacency fails. The exam trap here is assuming that IP connectivity and matching area IDs guarantee OSPF adjacency. In reality, OSPF adjacency depends on multiple interface parameters, and MTU mismatch is a subtle but critical factor. Practically, routers can ping each other because ICMP packets are smaller and less sensitive to MTU differences, but OSPF adjacency fails because OSPF packets during database exchange are larger and require matching MTU. Understanding this nuance is essential for CCNA candidates to diagnose adjacency issues effectively.
KKey Concepts to Remember
- OSPF routers must have matching MTU values on connected interfaces to successfully form neighbor adjacencies and exchange routing information.
- OSPF adjacency formation involves multiple parameters including area ID, hello/dead intervals, authentication, and MTU size, all of which must align.
- An MTU mismatch prevents OSPF routers from completing the database exchange phase, causing adjacency to remain in the INIT or EXSTART state.
- Basic IP connectivity and correct subnetting do not guarantee OSPF adjacency if interface parameters like MTU differ between neighbors.
- OSPF hello packets verify neighbor presence, but the adjacency fully forms only after successful database synchronization, which MTU mismatch can block.
- Routers do not need identical hostnames for OSPF adjacency; hostname differences do not affect OSPF neighbor relationships.
- OSPF operates over routed interfaces and does not require VLAN 1 or trunk configurations to form adjacencies on point-to-point links.
- Understanding MTU mismatch as a cause of adjacency failure helps avoid misdiagnosing OSPF neighbor issues that appear as connectivity problems.
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 MTU values on connected interfaces to successfully form neighbor adjacencies and exchange routing information.
What is the correct answer to this question?
The correct answer is: The OSPF MTU values do not match on the connected interfaces. — The most likely cause is an MTU mismatch between the two interfaces. In practical terms, the routers can still ping each other and appear correctly configured for OSPF at first glance, but adjacency formation involves more than basic IP connectivity. During the database exchange process, OSPF expects key interface characteristics to align, and MTU is one of the values that can stop the relationship from progressing normally. This is a classic CCNA troubleshooting scenario because the problem is subtle. The interfaces are up, the subnet is correct, and the area matches. But OSPF neighbors can still get stuck during formation if one side expects larger packets than the other side is willing to process. That makes the MTU mismatch the strongest answer here.
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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