Question 335 of 2,152

VRF-Lite →hardMultiple ChoiceObjective-mapped

Quick Answer

The answer is the 'ip mtu' command configured on one interface with a value lower than 1500, causing the DBD packet to be larger than the receiving interface's IP MTU. This is correct because OSPF sets the size of its Database Description packets based on the outgoing interface’s IP MTU, not the physical MTU; if the receiving interface has a smaller IP MTU, the oversized DBD packet is silently dropped, freezing the neighbor in EXSTART/EXCHANGE. On the Cisco CCNP ENARSI 300-410 exam, this question tests your understanding that an OSPF neighbor stuck in EXSTART due to an IP MTU mismatch in VRF-Lite is a classic trap—engineers often check only the physical MTU (e.g., 1500) and miss the 'ip mtu' command, which overrides it at Layer 3. Remember: OSPF cares about the IP MTU, not the interface MTU; if the neighbor won’t leave EXSTART, think "ip mtu" before you blame the physical link.

300-410 VRF-Lite Practice Question

This 300-410 practice question tests your understanding of vrf-lite. 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.

A network engineer configures VRF-Lite with OSPF as the routing protocol. Two routers in the same VRF are directly connected, but the OSPF neighbor state remains stuck in EXSTART/EXCHANGE. The engineer verifies that the MTU on both interfaces is 1500. 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.

Question 1hardmultiple choice

Review the full OSPF breakdown →

A
The OSPF network type is set to point-to-multipoint on one side and broadcast on the other, causing a mismatch in DBD packet size.
Why wrong: Network type mismatch can cause adjacency issues, but it typically results in a state of DOWN or ATTEMPT, not EXSTART/EXCHANGE.
B
The 'ip mtu' command is configured on one interface with a value lower than 1500, causing the DBD packet to be larger than the receiving interface's IP MTU.
OSPF uses the IP MTU for DBD packets. If one interface has a lower IP MTU (e.g., 1400), the DBD packet from the other side (1500) is dropped, causing the stuck state.
C
The VRF forwarding table is missing the OSPF route for the neighbor's router ID, preventing the exchange of DBD packets.
Why wrong: OSPF adjacency is established at Layer 2; the VRF routing table does not affect the DBD exchange process.
D
The OSPF dead interval is set to 40 seconds on one router and 120 seconds on the other, causing a mismatch in hello parameters.
Why wrong: Hello/dead interval mismatch would prevent the neighbor state from progressing beyond INIT or 2WAY, not EXSTART/EXCHANGE.

Full breakdown with real-world context →

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 'ip mtu' command is configured on one interface with a value lower than 1500, causing the DBD packet to be larger than the receiving interface's IP MTU.

OSPF uses the IP MTU of the outgoing interface to set the size of Database Description (DBD) packets. If the receiving interface has a smaller IP MTU, the DBD packet is silently dropped, causing the neighbor to stay in EXSTART/EXCHANGE. Even if the physical MTU is 1500, the IP MTU can be lowered via 'ip mtu' command, and OSPF will use that lower value. The MTU mismatch is not necessarily the physical MTU but the IP MTU, which is a common edge case.

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.

✗
The OSPF network type is set to point-to-multipoint on one side and broadcast on the other, causing a mismatch in DBD packet size.
Why it's wrong here
Network type mismatch can cause adjacency issues, but it typically results in a state of DOWN or ATTEMPT, not EXSTART/EXCHANGE.
✓
The 'ip mtu' command is configured on one interface with a value lower than 1500, causing the DBD packet to be larger than the receiving interface's IP MTU.
Why this is correct
OSPF uses the IP MTU for DBD packets. If one interface has a lower IP MTU (e.g., 1400), the DBD packet from the other side (1500) is dropped, causing the stuck state.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
OSPF neighbours must agree on key parameters.
✗
The VRF forwarding table is missing the OSPF route for the neighbor's router ID, preventing the exchange of DBD packets.
Why it's wrong here
OSPF adjacency is established at Layer 2; the VRF routing table does not affect the DBD exchange process.
✗
The OSPF dead interval is set to 40 seconds on one router and 120 seconds on the other, causing a mismatch in hello parameters.
Why it's wrong here
Hello/dead interval mismatch would prevent the neighbor state from progressing beyond INIT or 2WAY, not EXSTART/EXCHANGE.

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.

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 300-410 OSPF questions on adjacency and route selection.

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FAQ

Questions learners often ask

What does this 300-410 question test?

VRF-Lite — This question tests VRF-Lite — OSPF neighbours must agree on key parameters..

What is the correct answer to this question?

The correct answer is: The 'ip mtu' command is configured on one interface with a value lower than 1500, causing the DBD packet to be larger than the receiving interface's IP MTU. — OSPF uses the IP MTU of the outgoing interface to set the size of Database Description (DBD) packets. If the receiving interface has a smaller IP MTU, the DBD packet is silently dropped, causing the neighbor to stay in EXSTART/EXCHANGE. Even if the physical MTU is 1500, the IP MTU can be lowered via 'ip mtu' command, and OSPF will use that lower value. The MTU mismatch is not necessarily the physical MTU but the IP MTU, which is a common edge case.

What should I do if I get this 300-410 question wrong?

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?

OSPF neighbours must agree on key parameters.

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