Question 1hardmultiple choice
Full question →hardmultiple choiceObjective-mapped
Exhibit
R1 Gi0/0: ip address 10.10.12.1 255.255.255.252 ip ospf 10 area 0 ip mtu 1500 R2 Gi0/0: ip address 10.10.12.2 255.255.255.252 ip ospf 10 area 0 ip mtu 1400 Both interfaces are up/up. show ip ospf neighbor on both routers: Neighbor ID Pri State Dead Time Address Interface 2.2.2.2 1 EXSTART 00:00:31 10.10.12.2 Gi0/0
R1 and R2 should form an OSPF adjacency on their shared GigabitEthernet link, but they remain stuck in EXSTART. 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.
Both sides are in area 0, so area mismatch is not the problem.
B
Best answer
The interface MTU values do not match.
This is the classic cause of OSPF neighbors getting stuck in EXSTART or EXCHANGE.
C
Distractor review
One side is configured as passive-interface.
A passive interface would stop hello packets and usually prevent the neighbor from appearing like this.
D
Distractor review
The subnet mask on the link is incorrect.
Both ends are in the same /30, so the addressing shown is consistent.
Common exam trap
Common exam trap: answer the scenario, not the keyword
A common exam trap is selecting area mismatch or passive-interface as the cause of OSPF adjacency stuck in ExStart. While area mismatch prevents neighbor formation entirely, and passive-interface stops hello packets, these issues cause earlier failures, not ExStart stalls. The ExStart state specifically involves negotiating database description packets, which requires matching MTU values. Candidates often overlook MTU mismatches because neighbors appear in the topology, misleading them to suspect other configuration errors. Understanding that MTU mismatch allows neighbor discovery but blocks database synchronization 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 forms neighbor adjacencies to exchange routing information. The adjacency process includes several states: Down, Init, Two-Way, ExStart, Exchange, Loading, and Full. The ExStart state is where routers negotiate the master-slave relationship and sequence numbers for Database Description (DBD) packets. A critical factor in this negotiation is the interface MTU (Maximum Transmission Unit), which must match on both ends to ensure proper packet exchange. When OSPF neighbors have mismatched MTU values on their shared interface, they can discover each other and progress through initial states but get stuck in ExStart or Exchange. This happens because the routers cannot agree on the size of DBD packets, preventing the adjacency from reaching the Full state. Cisco routers strictly enforce MTU matching to avoid fragmentation and ensure reliable OSPF database synchronization. The exam trap here is assuming that adjacency failure is due to area mismatch, passive interface, or subnet mask issues, which typically prevent neighbor discovery or initial hello exchanges. In practice, MTU mismatches allow neighbors to see each other but block database synchronization, causing the adjacency to stall in ExStart. Network engineers must verify and align MTU settings on both ends of the link to resolve this issue and achieve full OSPF adjacency.
KKey Concepts to Remember
- OSPF routers must have matching interface MTU values to successfully negotiate Database Description packets during adjacency formation.
- An OSPF adjacency stuck in the ExStart state usually indicates a problem with DBD packet negotiation, often caused by MTU mismatches.
- OSPF neighbors in different areas cannot form adjacencies, but area mismatch typically prevents neighbor discovery before ExStart.
- A passive-interface configuration stops OSPF hello packets, preventing neighbor formation rather than causing adjacency to stall in ExStart.
- Subnet mask mismatches prevent routers from recognizing each other as neighbors, blocking adjacency formation at the hello stage.
- OSPF adjacency states progress sequentially, and failure to move beyond ExStart usually points to issues with packet size negotiation or authentication.
- Cisco routers enforce strict MTU matching on OSPF interfaces to avoid fragmentation and ensure reliable link-state database synchronization.
- Troubleshooting OSPF adjacency issues requires verifying interface parameters like MTU, area ID, and passive-interface status to isolate the root cause.
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 interface MTU values to successfully negotiate Database Description packets during adjacency formation.
What is the correct answer to this question?
The correct answer is: The interface MTU values do not match. — The MTU values do not match. OSPF neighbors can discover each other and even move through earlier states, but an MTU mismatch commonly leaves them stuck in EXSTART or EXCHANGE because the routers do not agree on database description packet sizing. Area mismatch, network mismatch, and passive-interface issues usually prevent a much earlier stage of adjacency formation.
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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