350-601 Network Practice Question
Exhibit
show ip ospf neighbor Neighbor ID Pri State Dead Time Address Interface 10.1.1.1 1 FULL/DR 00:00:38 192.168.1.2 Ethernet1/1 10.1.1.2 1 2WAY/DROTHER 00:00:34 192.168.1.3 Ethernet1/1 10.1.1.3 1 EXSTART/DR 00:00:32 192.168.1.4 Ethernet1/1
Refer to the exhibit. What is the most likely cause of neighbor 10.1.1.3 being stuck in EXSTART?
⚠ Common exam trap
Cisco often tests the MTU mismatch trap by having candidates confuse it with a network type mismatch, but the key differentiator is that MTU issues cause the adjacency to stall specifically in EXSTART/EXCHANGE, while network type mismatches prevent the adjacency from forming past INIT/2WAY.
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 between the interfaces.
In OSPF, the EXSTART state indicates that neighbors are negotiating the master/slave relationship and exchanging Database Description (DBD) packets. If the MTU of the interface on one side is larger than the MTU on the other, the larger DBD packet will be silently dropped, preventing the neighbor from progressing past EXSTART. This is a classic symptom of an MTU mismatch, as the OSPF adjacency will remain stuck in EXSTART or EXCHANGE.
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 ID.
Why it's wrong here
Duplicate RID would cause a neighbor to go down or be rejected.
- ✗
OSPF network type mismatch.
Why it's wrong here
Network type mismatch prevents DR election, but not typically EXSTART.
- ✓
MTU mismatch between the interfaces.
Why this is correct
MTU mismatch prevents DBD packets from being sent successfully.
- ✗
The interface is configured as passive.
Why it's wrong here
Passive interfaces do not form adjacencies.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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