The answer is an interface MTU mismatch between the two routers. When OSPF neighbors become stuck in the EXSTART state, the issue is almost always that the Database Description (DBD) packets are being rejected because the MTU values on the interfaces do not match. During EXSTART, the routers negotiate the master/slave relationship and exchange DBD packet headers, but if one router’s interface MTU is smaller than the size of the DBD packet it receives, the packet is silently dropped, preventing the exchange from progressing to the Loading state. On the CCNA 200-301 v2 exam, this scenario often appears with a serial link or a point-to-point connection, where a common trap is to blame a mismatched subnet mask or authentication error—but remember, those issues typically cause a neighbor to stall in INIT or 2WAY, not EXSTART. A solid memory tip is to think of EXSTART as the “exchange start” phase where the routers must agree on the packet size; if they can’t agree, they stay stuck, so always check the MTU first.
CCNA IP Routing Practice Question
This 200-301 practice question tests your understanding of ip routing. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. A key principle to apply: oSPF routers use a multi-step neighbor adjacency process including states like EXSTART and EXCHANGE to synchronize their link-state databases.. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
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?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
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.
Key principle: OSPF routers use a multi-step neighbor adjacency process including states like EXSTART and EXCHANGE to synchronize their link-state databases.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
An OSPF area mismatch
Why it's wrong here
Area mismatches usually prevent the relationship from progressing to a normal neighbor state at all.
When this WOULD be correct
In a different question setup where the focus is on OSPF neighbor relationships failing completely, an area mismatch could be presented as a potential cause for neighbors not forming at all. For example, if the question described routers in different OSPF areas trying to establish a neighbor relationship, this option would be correct.
✗
A duplicate router ID on R1 and R2
Why it's wrong here
A duplicate router ID causes instability, but EXSTART on a single link usually points elsewhere.
When this WOULD be correct
If the question were about OSPF neighbor relationships failing to establish due to configuration issues, and it explicitly mentioned that both routers have the same router ID, then option B would be the correct answer. This could occur in a scenario where both routers are configured with identical router IDs, leading to OSPF not forming any adjacency.
✓
An interface MTU mismatch between the routers
Why this is correct
EXSTART or EXCHANGE problems commonly occur when the MTU values do not match.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
OSPF routers use a multi-step neighbor adjacency process including states like EXSTART and EXCHANGE to synchronize their link-state databases.
In a different scenario, if the question specified that R2 was unable to route packets to R1 due to a lack of a default route, and the context involved troubleshooting routing issues rather than OSPF adjacency, then this option could be correct.
Option-by-option analysis
Why each answer is right or wrong
Understanding why wrong answers are wrong — and when they would be correct — is what separates a 750 score from a 900. The 200-301 exam frequently reuses these exact scenarios with slightly different constraints.
✓An interface MTU mismatch between the routersCorrect answer▾
Why this is correct
EXSTART or EXCHANGE problems commonly occur when the MTU values do not match.
✗An OSPF area mismatchWrong answer — click to see why▾
Why this is wrong here
An OSPF area mismatch prevents routers from forming a full adjacency; they typically remain in the INIT or 2-WAY state, not EXSTART. The EXSTART state indicates that the routers have already exchanged Hello packets and are attempting to negotiate the master/slave relationship, which requires matching area IDs.
★ When this WOULD be the correct answer
In a different question setup where the focus is on OSPF neighbor relationships failing completely, an area mismatch could be presented as a potential cause for neighbors not forming at all. For example, if the question described routers in different OSPF areas trying to establish a neighbor relationship, this option would be correct.
Why candidates choose this
Students often associate OSPF neighbor issues with area mismatches because it is a common configuration error. However, the specific state (EXSTART) points to a different problem, such as MTU mismatch, rather than area mismatch.
✗A duplicate router ID on R1 and R2Wrong answer — click to see why▾
Why this is wrong here
Duplicate router IDs cause OSPF to behave unpredictably, often resulting in flapping adjacencies or multiple neighbors with the same ID, but they do not typically cause a stuck EXSTART state on a single link. The routers would still progress through the states, but the adjacency may be unstable.
★ When this WOULD be the correct answer
If the question were about OSPF neighbor relationships failing to establish due to configuration issues, and it explicitly mentioned that both routers have the same router ID, then option B would be the correct answer. This could occur in a scenario where both routers are configured with identical router IDs, leading to OSPF not forming any adjacency.
Why candidates choose this
Duplicate router ID is a well-known OSPF issue, and test-takers may assume it can cause any neighbor problem. However, the EXSTART state is specifically related to the Database Description (DD) packet exchange, which is not directly affected by duplicate IDs.
✗A missing default route on R2Wrong answer — click to see why▾
Why this is wrong here
A missing default route on R2 does not affect OSPF adjacency formation. OSPF neighbors exchange routing information using multicast Hello and DD packets, which do not require a default route. The adjacency process is independent of the routing table content.
★ When this WOULD be the correct answer
In a different scenario, if the question specified that R2 was unable to route packets to R1 due to a lack of a default route, and the context involved troubleshooting routing issues rather than OSPF adjacency, then this option could be correct.
Why candidates choose this
Students may confuse the need for a default route for general network connectivity with the specific requirements for OSPF neighbor establishment. Since OSPF is a dynamic routing protocol, it does not rely on default routes to form adjacencies.
Analysis generated from the official 200-301blueprint and verified against question context. The “when correct” sections are what AI assistants cite when candidates ask “what’s the difference between these options?”
Common exam traps
Common exam trap: answer the scenario, not the keyword
Be aware that MTU mismatches cause EXSTART issues, while other mismatches prevent adjacency formation.
Detailed technical explanation
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.
Key takeaway
OSPF routers use a multi-step neighbor adjacency process including states like EXSTART and EXCHANGE to synchronize their link-state databases.
Real-world example
How this comes up in practice
A network engineer at a university connects two campus buildings via a fibre link. Both routers run OSPF, but no adjacency forms — even though both routers can ping each other. The engineer finds one router is in area 0 and the other in area 1. OSPF adjacency requires matching area numbers, hello/dead timers, and network type. IP reachability alone is not enough.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 200-301 question in full detail.
Review oSPF routers use a multi-step neighbor adjacency process including states like EXSTART and EXCHANGE to synchronize their link-state databases., then practise related 200-301 questions on the same topic to reinforce the concept.
IP Routing — This question tests IP Routing — 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?
Review oSPF routers use a multi-step neighbor adjacency process including states like EXSTART and EXCHANGE to synchronize their link-state databases., then practise related 200-301 questions on the same topic to reinforce the concept.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
OSPF routers use a multi-step neighbor adjacency process including states like EXSTART and EXCHANGE to synchronize their link-state databases.
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