- A
Check and adjust the MTU on the serial interfaces to match.
An MTU mismatch causes DBD packets to be dropped or fragmented, preventing OSPF from progressing past EXSTART/EXCHANGE. Setting the same MTU on both sides resolves the issue.
- B
Check and adjust the OSPF hello and dead timers to match.
Why wrong: This is incorrect because hello/dead timer mismatches cause OSPF to get stuck in INIT or 2-WAY, not EXSTART/EXCHANGE.
- C
Check and adjust the OSPF network type on the interfaces to match.
Why wrong: This is incorrect because network type mismatches typically cause OSPF to get stuck in INIT or prevent neighbor formation entirely, not EXSTART/EXCHANGE.
- D
Check and adjust the OSPF area ID on the interfaces to match.
Why wrong: This is incorrect because area ID mismatches prevent OSPF neighbors from forming at all; they will not reach EXSTART/EXCHANGE.
OSPF Adjacency Stuck in EXSTART/EXCHANGE
This 200-301 practice question tests your understanding of ip routing. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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.
You are connected to R1 via console. R1 and R2 are connected via two serial links: Serial0/0/0 (10.0.0.1/30) and Serial0/0/1 (10.0.0.5/30). OSPF is configured on both links. However, the OSPF neighbor adjacency is stuck in EXSTART/EXCHANGE state. You suspect a mismatch in OSPF parameters. You need to identify and fix the issue.
Quick Answer
The answer is to check and adjust the MTU on the serial interfaces to match. An OSPF adjacency stuck in EXSTART/EXCHANGE is a classic symptom of an MTU mismatch, because during the database description (DBD) packet exchange, OSPF sets the "I" (Init), "M" (More), and "MS" (Master/Slave) bits. If one interface has a lower MTU, the larger DBD packets are silently dropped or rejected, preventing the routers from moving past the EXSTART/EXCHANGE state and exchanging LSAs. On the CCNA 200-301 v2 exam, this scenario tests your ability to differentiate between common OSPF neighbor issues—a common trap is to immediately suspect a network type mismatch or authentication error, but the stuck state in EXSTART/EXCHANGE specifically points to MTU. A quick memory tip: "EXSTART/EXCHANGE stuck? Check the MTU—it's the size that matters."
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
Check and adjust the MTU on the serial interfaces to match.
The adjacency stuck in EXSTART/EXCHANGE is often due to an MTU mismatch. If one interface has a lower MTU, the DBD packets may be fragmented or rejected, preventing the exchange of LSAs. Setting the same MTU on both sides resolves the issue.
Key principle: OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Check and adjust the MTU on the serial interfaces to match.
- ✗
Check and adjust the OSPF hello and dead timers to match.
Why it's wrong here
This is incorrect because hello/dead timer mismatches cause OSPF to get stuck in INIT or 2-WAY, not EXSTART/EXCHANGE.
- ✗
Check and adjust the OSPF network type on the interfaces to match.
Why it's wrong here
This is incorrect because network type mismatches typically cause OSPF to get stuck in INIT or prevent neighbor formation entirely, not EXSTART/EXCHANGE.
- ✗
Check and adjust the OSPF area ID on the interfaces to match.
Why it's wrong here
This is incorrect because area ID mismatches prevent OSPF neighbors from forming at all; they will not reach EXSTART/EXCHANGE.
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.
✓Check and adjust the MTU on the serial interfaces to match.Correct answer▾
Why this is correct
An MTU mismatch causes DBD packets to be dropped or fragmented, preventing OSPF from progressing past EXSTART/EXCHANGE. Setting the same MTU on both sides resolves the issue.
✗Check and adjust the OSPF hello and dead timers to match.Wrong answer — click to see why▾
Why this is wrong here
The specific factual error is that timer mismatches affect the neighbor discovery phase, not the database exchange phase.
Why candidates choose this
Candidates often confuse the symptoms of timer mismatches with MTU mismatches, as both involve OSPF neighbor issues.
✗Check and adjust the OSPF network type on the interfaces to match.Wrong answer — click to see why▾
Why this is wrong here
The specific factual error is that network type affects the election of DR/BDR and adjacency formation, but not the DBD exchange process.
Why candidates choose this
Candidates may think that network type affects all stages of OSPF adjacency, but it primarily impacts the earlier stages.
✗Check and adjust the OSPF area ID on the interfaces to match.Wrong answer — click to see why▾
Why this is wrong here
The specific factual error is that area ID mismatches cause OSPF to ignore hello packets, so the adjacency never progresses beyond DOWN.
Why candidates choose this
Candidates may think area ID mismatches could cause issues later in the adjacency process, but they actually prevent any neighbor relationship.
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?”
Common exam traps
Common exam trap: OSPF can fail even when IP connectivity looks correct
OSPF neighbour formation depends on matching areas, timers, network type, authentication and passive-interface behaviour. Do not choose an answer only because the devices can ping.
Detailed technical explanation
How to think about this question
OSPF questions usually test the details that control adjacency and route selection. Read the neighbour state, area, router ID and interface configuration before deciding what is wrong.
KKey Concepts to Remember
- OSPF neighbours must agree on key parameters.
- Router ID selection can affect neighbour relationships and LSDB output.
- OSPF cost influences the preferred path.
- A route can appear in OSPF information but not become the installed route.
TExam Day Tips
- Check area mismatch first when OSPF adjacency fails.
- Review passive interfaces when a network is advertised but no neighbour forms.
- Use show ip ospf neighbor and show ip route clues carefully.
Key takeaway
OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.
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.
Review OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related 200-301 OSPF questions on adjacency and route selection.
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FAQ
Questions learners often ask
What does this 200-301 question test?
IP Routing — This question tests IP Routing — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: Check and adjust the MTU on the serial interfaces to match. — The adjacency stuck in EXSTART/EXCHANGE is often due to an MTU mismatch. If one interface has a lower MTU, the DBD packets may be fragmented or rejected, preventing the exchange of LSAs. Setting the same MTU on both sides resolves the issue.
What should I do if I get this 200-301 question wrong?
Review OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related 200-301 OSPF questions on adjacency and route selection.
What is the key concept behind this question?
OSPF neighbours must agree on key parameters.
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Last reviewed: Jun 7, 2026
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