350-601 Network Practice Question
A network engineer is troubleshooting an OSPF adjacency issue between two Nexus switches. The neighbors are stuck in the EXSTART state. What is the most likely cause?
⚠ Common exam trap
Cisco often tests the MTU mismatch trap by making candidates confuse it with other OSPF adjacency issues, such as area ID or timer mismatches, which cause different stuck states (e.g., INIT or 2-WAY) rather than EXSTART.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
MTU mismatch
When OSPF neighbors are stuck in the EXSTART state, the most common cause is an MTU mismatch. In EXSTART, the routers exchange Database Description (DBD) packets, which must be no larger than the interface MTU. If one interface has a smaller MTU, the DBD packet is dropped, preventing the adjacency from progressing to the Exchange state. Cisco Nexus switches default to an MTU of 1500 bytes, but any mismatch will cause this issue.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Hold timer mismatch
Why it's wrong here
Hold timer mismatch causes adjacency to go down, not stuck in EXSTART.
- ✗
Incorrect area ID
Why it's wrong here
Area ID mismatch prevents neighbor discovery, not EXSTART.
- ✓
MTU mismatch
Why this is correct
MTU mismatch leads to DBD packet rejection, keeping the neighbor in EXSTART.
- ✗
Duplicate router ID
Why it's wrong here
Duplicate router IDs prevent adjacency formation entirely, not just EXSTART.
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 350-601 practice question is part of Courseiva's free Cisco certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 350-601 exam.