Question 1mediummultiple choice
Full question →mediummultiple choiceObjective-mapped
Exhibit
R1# show ip ospf neighbor Neighbor ID Pri State Dead Time Address Interface 2.2.2.2 0 EXSTART 00:00:31 10.1.12.2 Serial0/0/0
Exhibit: R1 shows an OSPF neighbor stuck in EXSTART with R2 on a serial 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
Distractor review
An OSPF area mismatch
Area mismatches usually prevent the relationship from progressing to a normal neighbor state at all.
B
Distractor review
A duplicate router ID on R1 and R2
A duplicate router ID causes instability, but EXSTART on a single link usually points elsewhere.
C
Best answer
An interface MTU mismatch between the routers
EXSTART or EXCHANGE problems commonly occur when the MTU values do not match.
D
Distractor review
A missing default route on R2
A default route is not required to form an OSPF adjacency.
Common exam trap
Common exam trap: answer the scenario, not the keyword
A common exam trap is to confuse an OSPF area mismatch or duplicate router ID with the EXSTART adjacency stall. Area mismatches prevent neighbor relationships from forming beyond the INIT or DOWN states, so the adjacency never reaches EXSTART. Duplicate router IDs cause instability and flapping but do not specifically cause the adjacency to remain stuck in EXSTART. Candidates often overlook the MTU mismatch because it is less obvious and requires checking interface parameters. Misdiagnosing the problem leads to incorrect troubleshooting steps and exam answers.
Technical deep dive
How to think about this question
OSPF (Open Shortest Path First) is a link-state routing protocol that establishes neighbor relationships through a series of states, including EXSTART and EXCHANGE. During EXSTART, routers elect a master and slave to control the exchange of Database Description (DBD) packets, which summarize the link-state database. This process requires both routers to agree on interface parameters such as MTU to ensure the DBD packets are accepted and processed correctly. When two OSPF routers have an interface MTU mismatch, the DBD packets sent during the EXSTART and EXCHANGE states are rejected because the MTU field in the packets does not match the receiving router's interface MTU. This mismatch prevents the routers from progressing beyond the EXSTART state, causing the adjacency to become stuck. Cisco routers use the MTU value as a critical parameter to maintain database consistency and prevent fragmentation or packet loss during OSPF exchanges. The EXSTART state stall caused by MTU mismatch is a common exam trap because candidates might incorrectly suspect area mismatches or router ID conflicts. However, area mismatches prevent adjacency formation earlier, and router ID conflicts cause instability rather than a stuck EXSTART state. In practical networks, ensuring consistent MTU settings on serial or other point-to-point interfaces is essential to avoid OSPF adjacency issues and maintain stable routing.
KKey Concepts to Remember
- OSPF routers use a multi-step neighbor adjacency process including states like EXSTART and EXCHANGE to synchronize their link-state databases.
- An interface MTU mismatch between OSPF neighbors prevents successful Database Description (DBD) packet exchange, causing the adjacency to stall in EXSTART.
- OSPF neighbors must agree on interface parameters such as MTU to progress beyond EXSTART and complete the link-state database synchronization.
- Area mismatches in OSPF typically prevent neighbor relationships from forming beyond the INIT or DOWN states, not causing EXSTART stalls.
- Duplicate router IDs cause OSPF instability and flapping but do not specifically cause neighbors to remain stuck in EXSTART state.
- A missing default route on a router does not affect OSPF neighbor adjacency formation or progression through neighbor states.
- OSPF Database Description packets include MTU information, and mismatched MTU values cause these packets to be rejected, halting adjacency progress.
- Serial links in OSPF require matching MTU settings on both ends to ensure successful neighbor adjacency and database synchronization.
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 use a multi-step neighbor adjacency process including states like EXSTART and EXCHANGE to synchronize their link-state databases.
What is the correct answer to this question?
The correct answer is: An interface MTU mismatch between the routers — When two OSPF routers stay in EXSTART, the first thing to suspect is an MTU mismatch. They can discover each other, but database exchange does not complete because the DBD packets do not agree on interface MTU.
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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