hardMultiple ChoiceObjective-mapped
OSPF MTU Mismatch: Stuck in EXSTART/EXCHANGE
An engineer configures OSPF on two directly connected routers with MTU 1500 on one interface and MTU 1600 on the other. The OSPF adjacency remains stuck in EXSTART/EXCHANGE state. Which is the most likely explanation?
Quick Answer
The answer is that the larger MTU interface sends DBD packets that are too big for the smaller MTU interface, preventing the exchange of LSAs. This occurs because OSPF embeds the interface MTU into Database Description packets, and a router receiving a DBD packet larger than its own configured MTU will silently drop it, stalling the adjacency in EXSTART/EXCHANGE. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding that OSPF does not negotiate MTU—it simply enforces it, making MTU mismatch a classic trap when both sides are not explicitly set to the same value. Many engineers mistakenly assume OSPF will fragment or adapt, but the protocol requires identical MTU for DBD exchange to proceed. A reliable memory tip: "MTU must match, or EXSTART will catch."
⚠ Common exam trap
Cisco often tests the misconception that OSPF uses TCP or that MTU mismatches affect hello packets, when in fact the problem only surfaces during DBD exchange in the EXSTART/EXCHANGE state.
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
✓
The larger MTU interface sends DBD packets that are too big for the smaller MTU interface, preventing the exchange of LSAs.
OSPF uses IP protocol 89 and directly exchanges Database Description (DBD) packets during the EXSTART/EXCHANGE state. When the MTU on one interface is smaller (1500) than the DBD packet size sent by the neighbor with the larger MTU (1600), the smaller interface drops the oversized packet. This prevents the successful exchange of LSAs, causing the adjacency to remain stuck in EXSTART/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.
- ✓
The larger MTU interface sends DBD packets that are too big for the smaller MTU interface, preventing the exchange of LSAs.
Why this is correct
OSPF DBD packets are limited by the outgoing interface MTU; a mismatch causes the smaller MTU router to drop the packets, stalling the adjacency.
- ✗
OSPF uses TCP, and the MSS mismatch causes the adjacency to fail.
Why it's wrong here
OSPF uses IP protocol 89, not TCP, so MSS is irrelevant.
- ✗
The MTU mismatch causes a routing loop that prevents the exchange of hello packets.
Why it's wrong here
MTU mismatch does not affect hello packets; they are small and always fit within any MTU.
- ✗
The router with the smaller MTU will not send hello packets because it detects the mismatch.
Why it's wrong here
Hello packets are not affected by MTU mismatch; the issue is with DBD packets.
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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Same concept, more angles
1 more way this is tested on 300-410
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. 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?
medium- A.OSPF adjacency is forming correctly, moving to FULL state.
- ✓ B.OSPF adjacency is stuck due to MTU mismatch; the neighbor has a larger MTU.
- C.OSPF adjacency is stuck because the router is not the DR/BDR.
- D.OSPF adjacency is stuck due to authentication mismatch.
Why B: 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.
JA
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.