350-501 Networking Practice Question
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?
⚠ Common exam trap
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.
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
✓
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.
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.
- ✗
Duplicate router IDs.
Why it's wrong here
Duplicate router IDs can cause unpredictable behavior but not typically EXSTART state.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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