The answer is a mismatched MTU between the two routers. On a point-to-point OSPF link, the EXSTART state is where routers negotiate the master/slave relationship for Database Description (DBD) packet exchange; if the MTU values differ, the larger DBD packets are silently dropped or rejected by the interface with the smaller MTU, preventing the adjacency from progressing past EXSTART. This scenario is a classic trap on the Cisco SPCOR 350-501 exam, often presented with a serial link where one side has a default MTU of 1500 and the other has been manually lowered, testing your understanding that OSPF will not proceed to the Exchange state until DBD packets are successfully acknowledged. A common memory tip is to think of EXSTART as the “handshake” phase—if the handshake fails because the packets are too big, the adjacency stays stuck. Remember: MTU mismatch kills DBD exchange, so always verify with “show ip ospf interface” to compare the MTU values on both ends.
350-501 Networking Practice Question
This 350-501 practice question tests your understanding of networking. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. 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.
A network engineer is troubleshooting an OSPF adjacency issue between two routers connected via a serial link. The routers are configured with point-to-point network type. The adjacency stays in EXSTART state. 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
✓
Mismatched MTU.
On a point-to-point OSPF link, the EXSTART state indicates that the routers have formed a bidirectional communication (2WAY) and are now negotiating the master/slave relationship for database description (DBD) packet exchange. A mismatched MTU between the two interfaces causes the DBD packets to be dropped or rejected, preventing the adjacency from progressing beyond EXSTART. This is a common issue on serial links where one side may have a different MTU configured.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
Mismatched OSPF area ID.
Why it's wrong here
Area ID mismatch prevents the adjacency from even becoming INIT.
✗
Mismatched hello/dead intervals.
Why it's wrong here
Mismatched intervals prevent negotiation, resulting in DOWN state.
✓
Mismatched MTU.
Why this is correct
OSPF uses the MTU in Database Description packets; a mismatch causes the adjacency to stay in EXSTART.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
Duplicate router IDs.
Why it's wrong here
Duplicate router IDs can cause unpredictable behavior but not typically EXSTART state.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the EXSTART state trap by making candidates confuse it with the more common mismatched hello/dead intervals or area ID issues, which actually prevent reaching EXSTART, while MTU mismatch is the specific cause of stalling in EXSTART.
Detailed technical explanation
How to think about this question
In OSPF, the EXSTART state is where the master/slave election occurs using DBD packets that include the MTU field. If the receiving router's interface MTU is smaller than the DBD packet size, the packet is silently dropped (on Cisco IOS, the 'ip ospf mtu-ignore' command can bypass this check). This behavior is defined in RFC 2328, Section 10.6, and is a frequent cause of adjacency issues on serial links with mismatched MTU values.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
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
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Networking — This question tests Networking — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Mismatched MTU. — On a point-to-point OSPF link, the EXSTART state indicates that the routers have formed a bidirectional communication (2WAY) and are now negotiating the master/slave relationship for database description (DBD) packet exchange. A mismatched MTU between the two interfaces causes the DBD packets to be dropped or rejected, preventing the adjacency from progressing beyond EXSTART. This is a common issue on serial links where one side may have a different MTU configured.
What should I do if I get this 350-501 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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?
Read the scenario before looking for a memorised answer.
About these practice questions
Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. Refer to the exhibit. The router cannot form a full OSPF adjacency with 10.0.0.2. Which is the most likely cause?
medium
A.Duplicate router IDs.
B.Mismatched authentication.
C.Mismatched hello interval.
✓ D.Mismatched MTU.
Why D: Option A is correct because a mismatched MTU commonly causes OSPF to get stuck in EXSTART state. Option B (hello interval) would prevent INIT. Option C (duplicate router ID) might cause other issues. Option D (authentication) would cause failure earlier.
Last reviewed: Jun 25, 2026
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