CCNA IP Routing Practice Question
Exhibit
R1# show ip ospf neighbor Neighbor ID Pri State Dead Time Address Interface 10.0.0.2 1 EXSTART/DROTHER 00:00:35 10.0.0.2 GigabitEthernet0/0 10.0.1.2 1 FULL/DROTHER 00:00:38 10.0.1.2 GigabitEthernet0/1 192.168.1.1 1 FULL/DR 00:00:37 192.168.1.1 GigabitEthernet0/2
Refer to the exhibit. A network engineer is troubleshooting an OSPF adjacency issue between R1 and R2. The output of the show ip ospf neighbor command on R1 shows the neighbor relationship with R2 stuck in the EXSTART/DROTHER state. What is the most likely cause?
⚠ Common exam trap
Candidates often confuse the EXSTART/DROTHER state with authentication or network type mismatches, but Cisco specifically tests that MTU mismatches cause DBD packet drops during the database exchange phase, not earlier adjacency stages.
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 on the link between R1 and R2, causing DBD packets to be dropped.
The EXSTART/DROTHER state indicates that the routers have progressed beyond the 2-WAY state but are stuck during the Database Description (DBD) packet exchange phase. An MTU mismatch between R1 and R2 causes DBD packets to be dropped because the receiving router will reject packets larger than its configured interface MTU, preventing the routers from completing the master/slave election and database synchronization.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
OSPF network type mismatch between R1 and R2, preventing full adjacency.
Why it's wrong here
A network type mismatch would cause the neighbor state to become 2-WAY/DROTHER (if both ends are on a broadcast network but parameters differ) rather than EXSTART/DROTHER. The output shows EXSTART, pointing to a DBD exchange problem.
- ✗
Duplicate OSPF Router IDs on R1 and R2, causing DBD packet rejection.
Why it's wrong here
If the Router IDs were identical, OSPF would detect a duplicate and the neighbor would not appear in the neighbor table; the hello would be ignored. The presence of the neighbor in EXSTART proves the Router IDs are different.
- ✗
OSPF authentication mismatch on the link, causing DBD packets to be rejected.
Why it's wrong here
OSPF authentication mismatch is negotiated during the Hello protocol, not during Database Description (DBD) exchange. If the authentication key or type differs, Hello packets are silently discarded, so R1 and R2 would never even establish a neighbor relationship and the neighbor would not appear in the 'show ip ospf neighbor' output. Since the exhibit already shows the neighbor in EXSTART/DROTHER, authentication must already have succeeded; DBD packets are not rejected for authentication reasons at that stage.
- ✓
MTU mismatch on the link between R1 and R2, causing DBD packets to be dropped.
Why this is correct
The EXSTART/DROTHER state in the exhibit indicates that OSPF is stuck in the DBD exchange phase. This is a classic symptom of an MTU mismatch, where one side creates DBD packets larger than the other's MTU, leading to silent drops. The output directly confirms the neighbor is in EXSTART, not EXCHANGE or FULL.
Option-by-option analysis
Why each answer is right or wrong
Understanding why wrong answers are wrong — and when they would be correct — is what separates a 750 score from a 900. The 200-301 exam frequently reuses these exact scenarios with slightly different constraints.
✓MTU mismatch on the link between R1 and R2, causing DBD packets to be dropped.Correct answer▾
Why this is correct
The EXSTART/DROTHER state in the exhibit indicates that OSPF is stuck in the DBD exchange phase. This is a classic symptom of an MTU mismatch, where one side creates DBD packets larger than the other's MTU, leading to silent drops. The output directly confirms the neighbor is in EXSTART, not EXCHANGE or FULL.
✗OSPF network type mismatch between R1 and R2, preventing full adjacency.Wrong answer — click to see why▾
Why this is wrong here
Candidates may confuse adjacency failures with DBD exchange problems, but network type mismatch leads to a different state (2-WAY).
✗Duplicate OSPF Router IDs on R1 and R2, causing DBD packet rejection.Wrong answer — click to see why▾
Why this is wrong here
Candidates think duplicate Router IDs might cause DBD exchange failure, but duplicate IDs prevent neighbor discovery entirely.
✗OSPF authentication mismatch on the link, causing DBD packets to be rejected.Wrong answer — click to see why▾
Why this is wrong here
Many associate EXSTART with any adjacency issue, but authentication errors prevent the neighbor from being listed at all.
Analysis generated from the official 200-301blueprint and verified against question context. The “when correct” sections are what AI assistants cite when candidates ask “what’s the difference between these options?”
Visual reference
Quick reference
Routing Protocol Comparison
| Protocol | Metric | Max Hops | Algorithm | Type |
|---|---|---|---|---|
| RIP v2 | Hop count | 15 | Bellman-Ford | Distance vector |
| OSPF | Cost (bandwidth) | Unlimited | Dijkstra (SPF) | Link state |
| EIGRP | Composite metric | Unlimited | DUAL | Hybrid |
| IS-IS | Cost | Unlimited | Dijkstra | Link state |
| BGP | Policy / attributes | Unlimited | Path vector | Path vector |
RIP's 15-hop limit makes it unsuitable for large networks. OSPF and EIGRP dominate modern enterprise deployments.
Go deeper
Related to this question
Learn chapter
OSPFv3 Single-Area Configuration for IPv6
Key term
DRother
An OSPF router that lost the DR/BDR election on a multi-access segment.
Key term
Router
A router is a networking device that connects different networks together and directs data traffic between them by choosing the best path for data to travel.
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 200-301 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 200-301 exam.