Question 578 of 2,152

MPLS Operations →hardMultiple ChoiceObjective-mapped

300-410 MPLS Operations Practice Question

This 300-410 practice question tests your understanding of mpls operations. 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.

A network engineer is troubleshooting MPLS traffic engineering (TE) tunnels. A TE tunnel from Router A to Router B is configured but remains down. The engineer runs show mpls traffic-eng tunnels and sees that the tunnel is in 'down' state with the error 'Path computation failed'. The IGP is OSPF with MPLS TE enabled, and the network is fully meshed. What is the most likely root cause?

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
MPLS TE is not enabled on all interfaces along the path.
Correct because MPLS TE must be enabled on each interface to advertise link attributes into the TED; otherwise, the headend cannot compute a constraint-based path.
B
The tunnel destination is not reachable via the IGP.
Why wrong: Incorrect because the tunnel destination is a TE tunnel headend; reachability is not required for TE path computation, only the TED.
C
RSVP is not configured on the tunnel interface.
Why wrong: Incorrect because RSVP is used for signaling, but the path computation fails before signaling; RSVP configuration is not the cause of computation failure.
D
The tunnel bandwidth is set too high, exceeding available bandwidth.
Why wrong: Incorrect because while bandwidth constraints can cause path computation failure, the error message 'Path computation failed' without specific bandwidth details suggests a more fundamental issue like missing TE configuration.

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

✓

MPLS TE is not enabled on all interfaces along the path.

The 'Path computation failed' error indicates that the headend router cannot find a path meeting the constraints. Since OSPF TE is enabled, the issue is likely that the traffic engineering database (TED) is incomplete because MPLS TE is not enabled on all intermediate interfaces, preventing the headend from learning link attributes.

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.

✓
MPLS TE is not enabled on all interfaces along the path.
Why this is correct
Correct because MPLS TE must be enabled on each interface to advertise link attributes into the TED; otherwise, the headend cannot compute a constraint-based path.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
OSPF neighbours must agree on key parameters.
✗
The tunnel destination is not reachable via the IGP.
Why it's wrong here
Incorrect because the tunnel destination is a TE tunnel headend; reachability is not required for TE path computation, only the TED.
✗
RSVP is not configured on the tunnel interface.
Why it's wrong here
Incorrect because RSVP is used for signaling, but the path computation fails before signaling; RSVP configuration is not the cause of computation failure.
✗
The tunnel bandwidth is set too high, exceeding available bandwidth.
Why it's wrong here
Incorrect because while bandwidth constraints can cause path computation failure, the error message 'Path computation failed' without specific bandwidth details suggests a more fundamental issue like missing TE configuration.

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?

MPLS Operations — This question tests MPLS Operations — OSPF neighbours must agree on key parameters..

What is the correct answer to this question?

The correct answer is: MPLS TE is not enabled on all interfaces along the path. — The 'Path computation failed' error indicates that the headend router cannot find a path meeting the constraints. Since OSPF TE is enabled, the issue is likely that the traffic engineering database (TED) is incomplete because MPLS TE is not enabled on all intermediate interfaces, preventing the headend from learning link attributes.

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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