- A
The larger MTU interface sends DBD packets that are too big for the smaller MTU interface, preventing the exchange of LSAs.
OSPF DBD packets are limited by the outgoing interface MTU; a mismatch causes the smaller MTU router to drop the packets, stalling the adjacency.
- B
OSPF uses TCP, and the MSS mismatch causes the adjacency to fail.
Why wrong: OSPF uses IP protocol 89, not TCP, so MSS is irrelevant.
- C
The MTU mismatch causes a routing loop that prevents the exchange of hello packets.
Why wrong: MTU mismatch does not affect hello packets; they are small and always fit within any MTU.
- D
The router with the smaller MTU will not send hello packets because it detects the mismatch.
Why wrong: Hello packets are not affected by MTU mismatch; the issue is with DBD packets.
OSPF MTU Mismatch: Stuck in EXSTART/EXCHANGE
This 300-410 practice question tests your understanding of device management. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
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?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue:
"most likely"Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
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."
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.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
OSPF uses TCP, and the MSS mismatch causes the adjacency to fail.
- ✗
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.
Common exam traps
Common exam trap: answer the scenario, not the keyword
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.
Detailed technical explanation
How to think about this question
The OSPF MTU mismatch issue is defined in RFC 2328, where the Database Description packet must not exceed the interface MTU. In practice, the 'ip ospf mtu-ignore' command can bypass this check, but it may lead to packet fragmentation or loss. A real-world scenario is when a GRE tunnel or VLAN trunking changes the effective MTU on one side without the operator noticing.
KKey Concepts to Remember
- Read the scenario before looking for a memorised answer.
- Find the constraint that changes the correct option.
- Eliminate answers that are true in general but not in this case.
TExam Day Tips
- Watch for words such as best, first, most likely and least administrative effort.
- Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A network engineer at a university connects two campus buildings via a fibre link. Both routers run OSPF, but no adjacency forms — even though both routers can ping each other. The engineer finds one router is in area 0 and the other in area 1. OSPF adjacency requires matching area numbers, hello/dead timers, and network type. IP reachability alone is not enough.
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.
What to study next
Got this wrong? Here's your next step.
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FAQ
Questions learners often ask
What does this 300-410 question test?
Device Management — This question tests Device Management — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: 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.
What should I do if I get this 300-410 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
About these practice questions
Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →
Same concept, more angles
1 more ways 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.
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Last reviewed: Jul 4, 2026
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