Question 350 of 2,152

NAT and PAT →hardMultiple ChoiceObjective-mapped

OSPF Neighbor Stuck in EXSTART Due to MTU Mismatch

This 300-410 practice question tests your understanding of nat and pat. 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 routers connected via a serial link. Both routers have 'ip ospf network point-to-point' configured, but the link is actually a Frame Relay multipoint subinterface. The OSPF neighbors remain stuck in EXSTART 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.

A
The MTU on the serial link is not consistent between the two routers, causing DBD packets to be dropped.
Correct. OSPF uses the interface MTU for DBD packets; mismatch prevents exchange.
B
The OSPF hello timer is set too high, causing the neighbor to time out.
Why wrong: Hello timer mismatch would cause a different state, not EXSTART.
C
The area ID is different on the two routers.
Why wrong: Area ID mismatch would prevent neighbor formation entirely, not stuck in EXSTART.
D
The OSPF router ID is duplicated.
Why wrong: Duplicate router ID would cause neighbor to flap, not stuck in EXSTART.

Full breakdown with real-world context →

Quick Answer

The answer is an MTU mismatch between the two routers, which causes OSPF Database Description (DBD) packets to be dropped during the EXSTART state. When OSPF neighbors attempt to negotiate the master/slave relationship and exchange DBD packet headers, the interface MTU determines the maximum packet size; if one router’s serial link has a lower MTU—common on Frame Relay multipoint subinterfaces—the larger DBD packets from the higher-MTU router are silently discarded, stalling the adjacency. This scenario tests your understanding of how OSPF network type and Layer 2 encapsulation interact, a frequent trap on the Cisco CCNP ENARSI 300-410 exam: candidates often blame the network type mismatch itself, but the root cause is the resulting MTU inconsistency. Remember the mnemonic “EXSTART Exchanges MTU” to recall that DBD drops in EXSTART almost always point to an MTU problem, not a misconfigured network type.

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 MTU on the serial link is not consistent between the two routers, causing DBD packets to be dropped.

When OSPF neighbors are stuck in EXSTART state, it typically indicates a problem with the Database Description (DBD) packet exchange. In this scenario, the 'ip ospf network point-to-point' command overrides the default Frame Relay multipoint behavior, but the actual link is a multipoint subinterface. This mismatch can cause MTU inconsistencies because the multipoint subinterface may have a different MTU than the point-to-point OSPF network type expects. If the MTU is not consistent between the two routers, DBD packets larger than the smaller MTU will be silently dropped, preventing the neighbors from progressing beyond EXSTART.

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 MTU on the serial link is not consistent between the two routers, causing DBD packets to be dropped.
Why this is correct
Correct. OSPF uses the interface MTU for DBD packets; mismatch prevents exchange.
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.
✗
The OSPF hello timer is set too high, causing the neighbor to time out.
Why it's wrong here
Hello timer mismatch would cause a different state, not EXSTART.
✗
The area ID is different on the two routers.
Why it's wrong here
Area ID mismatch would prevent neighbor formation entirely, not stuck in EXSTART.
✗
The OSPF router ID is duplicated.
Why it's wrong here
Duplicate router ID would cause neighbor to flap, not stuck in EXSTART.

Common exam traps

Common exam trap: answer the scenario, not the keyword

Cisco often tests the distinction between neighbor states—specifically that EXSTART is reached only after hello and 2-WAY states are complete, so issues like mismatched area IDs or duplicate router IDs would prevent reaching EXSTART, while MTU mismatches specifically block DBD exchange at EXSTART.

Detailed technical explanation

How to think about this question

OSPF uses DBD packets to exchange link-state database summaries during the EXSTART state. If the MTU is mismatched, a router will drop DBD packets that exceed the interface MTU, but OSPF does not automatically fragment these packets. This is a common issue on Frame Relay multipoint networks where subinterfaces may have different MTU values, and the 'ip ospf network point-to-point' command does not adjust the underlying physical interface MTU. In real-world scenarios, this can be resolved by manually setting the 'ip mtu' to match on both ends or by using 'ip ospf mtu-ignore' as a workaround.

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.

Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.

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FAQ

Questions learners often ask

What does this 300-410 question test?

NAT and PAT — This question tests NAT and PAT — Read the scenario before looking for a memorised answer..

What is the correct answer to this question?

The correct answer is: The MTU on the serial link is not consistent between the two routers, causing DBD packets to be dropped. — When OSPF neighbors are stuck in EXSTART state, it typically indicates a problem with the Database Description (DBD) packet exchange. In this scenario, the 'ip ospf network point-to-point' command overrides the default Frame Relay multipoint behavior, but the actual link is a multipoint subinterface. This mismatch can cause MTU inconsistencies because the multipoint subinterface may have a different MTU than the point-to-point OSPF network type expects. If the MTU is not consistent between the two routers, DBD packets larger than the smaller MTU will be silently dropped, preventing the neighbors from progressing beyond EXSTART.

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.

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