350-501 Networking Practice Question
Exhibit
show ip ospf neighbor Neighbor ID Pri State Dead Time Address Interface 10.0.0.2 1 EXSTART/DR 00:00:35 10.0.1.2 GigabitEthernet0/0
Refer to the exhibit. The router cannot form a full OSPF adjacency with 10.0.0.2. Which is the most likely cause?
⚠ Common exam trap
Cisco often tests the MTU mismatch scenario by showing a neighbor stuck in EXSTART state, leading candidates to mistakenly choose mismatched hello intervals or authentication, which would instead cause the neighbor to be stuck in DOWN or INIT.
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.
The most likely cause is a mismatched MTU because OSPF uses the interface MTU to determine whether to form an adjacency. If the MTU on one side is smaller than the size of the Database Description (DBD) packet, the neighbor will reject the packet and remain in the EXSTART state, preventing full adjacency. This is a common issue when connecting interfaces with different MTU values, such as a default 1500 bytes on one side and a lower value on the other.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Duplicate router IDs.
Why it's wrong here
Duplicate router IDs can cause confusion but not typically EXSTART state.
- ✗
Mismatched authentication.
Why it's wrong here
Authentication mismatch prevents adjacency; state would not reach EXSTART.
- ✗
Mismatched hello interval.
Why it's wrong here
Mismatched hello/dead intervals prevent adjacency from forming, staying in DOWN state.
- ✓
Mismatched MTU.
Why this is correct
OSPF Database Description packets include MTU; a mismatch prevents progression from EXSTART.
Visual reference
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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