The answer is that IGP TE extensions are not enabled on the head-end router. This is the most likely cause because MPLS TE path computation relies on a Traffic Engineering Database (TED), which is built from link attributes flooded by IGP TE extensions such as OSPF TE or IS-IS TE. Without these extensions, the head-end router lacks the necessary topology and bandwidth data to calculate a valid path, resulting in the "path computation failed" error. On the Cisco SPCOR 350-501 exam, this scenario tests your understanding of the MPLS TE control plane hierarchy: IGP provides the database, RSVP handles signaling, and LDP is irrelevant for TE. A common trap is confusing path computation with signaling failures—remember, RSVP errors show as "signaling failed," not "path computation failed." For a memory tip, think "No IGP TE, no TED, no path."
350-501 Networking Practice Question
This 350-501 practice question tests your understanding of networking. 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.
An engineer configures MPLS TE tunnels. After configuration, the tunnel remains down. The 'show mpls traffic-eng tunnels' output shows 'Tunnel is down - path computation failed'. What is the most likely 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.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
IGP TE extensions are not enabled on the head-end router.
Option A is correct because path computation failure often indicates that IGP TE extensions (e.g., IS-IS TE or OSPF TE) are not enabled, so routers do not have the required link attributes. Option B is incorrect; LDP is not required for TE. Option C is incorrect; RSVP is used for signaling but the issue is path computation, not signaling. Option D is plausible but less common; MTU mismatch would cause signaling issues, not path computation failure.
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.
✓
IGP TE extensions are not enabled on the head-end router.
Why this is correct
Correct. Without TE extensions, the router cannot compute a path.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Incorrect. RSVP is for signaling, but path computation occurs before signaling.
✗
MPLS LDP is not enabled on the head-end router.
Why it's wrong here
Incorrect. LDP is used for label distribution, not for TE path computation.
✗
There is an MTU mismatch along the path.
Why it's wrong here
Incorrect. MTU mismatch would cause signaling errors, not path computation failure.
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 350-501 OSPF questions on adjacency and route selection.
Networking — This question tests Networking — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: IGP TE extensions are not enabled on the head-end router. — Option A is correct because path computation failure often indicates that IGP TE extensions (e.g., IS-IS TE or OSPF TE) are not enabled, so routers do not have the required link attributes. Option B is incorrect; LDP is not required for TE. Option C is incorrect; RSVP is used for signaling but the issue is path computation, not signaling. Option D is plausible but less common; MTU mismatch would cause signaling issues, not path computation failure.
What should I do if I get this 350-501 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 350-501 OSPF questions on adjacency and route selection.
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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