mediumMultiple Choice
OSPF MTU Mismatch: Stuck in EXSTART/EXCHANGE
A network engineer runs the following command to troubleshoot an OSPF adjacency issue:
R1# debug ip ospf adj
OSPF: Interface GigabitEthernet0/0 going Up OSPF: Send with youngest orig age 0 OSPF: Rcv DBD from 2.2.2.2 seq 0x1A opt 0x52 flag 0x7 len 32 mtu 1500 state INIT OSPF: First DBD and we are not SLAVE OSPF: Rcv DBD from 2.2.2.2 seq 0x1A opt 0x52 flag 0x7 len 32 mtu 1500 state EXSTART OSPF: Nbr 2.2.2.2 has larger interface MTU
What does this output indicate?
⚠ Common exam trap
Cisco often tests the MTU mismatch trap by showing debug output with 'Nbr has larger interface MTU' and candidates mistakenly think the adjacency is progressing normally (Option A) or confuse it with DR/BDR election issues (Option C), but the key is that OSPF adjacency will not form until MTU values match.
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
✓
OSPF adjacency is stuck due to MTU mismatch; the neighbor has a larger MTU.
The debug output shows that R1 receives a Database Description (DBD) packet from neighbor 2.2.2.2 with the INIT and EXSTART states, and then explicitly states 'Nbr 2.2.2.2 has larger interface MTU'. This indicates an MTU mismatch where the neighbor's interface MTU (1500) is larger than R1's interface MTU, preventing the OSPF adjacency from progressing beyond the EXSTART state. Option B is correct because the adjacency is stuck due to the MTU mismatch, as OSPF requires matching MTU values on both sides for DBD exchange to complete.
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 adjacency is forming correctly, moving to FULL state.
Why it's wrong here
The debug shows EXSTART with the neighbour reporting a larger interface MTU, so the adjacency stalls rather than reaching FULL; mismatched MTU blocks DBD exchange. This output would indicate healthy progression only if both interfaces shared identical MTU values and the state advanced past EXSTART.
- ✓
OSPF adjacency is stuck due to MTU mismatch; the neighbor has a larger MTU.
Why this is correct
The debug line "Nbr 2.2.2.2 has larger interface MTU" directly reports the mismatch, so the adjacency stalls in EXSTART because DBD packets are rejected. The neighbour's larger MTU prevents OSPF database description exchange from completing.
- ✗
OSPF adjacency is stuck because the router is not the DR/BDR.
Why it's wrong here
DR/BDR status is irrelevant to EXSTART; the debug shows the neighbour has a larger interface MTU, so the master/slave negotiation stalls and DBD exchange cannot proceed. DR/BDR election occurs after two-way and does not block adjacency formation at EXSTART.
- ✗
OSPF adjacency is stuck due to authentication mismatch.
Why it's wrong here
The debug shows the neighbour reaching EXSTART then reporting a larger interface MTU, so the stuck adjacency stems from an MTU mismatch, not authentication. Authentication failures halt OSPF earlier, during hello exchange, producing 'authentication failure' messages; that is the scenario where this diagnosis would apply.
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.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 300-410 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 300-410 exam.