Quick Answer
The answer is that the explicit path contains a link that is down. When an MPLS TE tunnel is configured with an explicit path, the headend router uses RSVP to signal the path hop by hop, verifying that each link is operational before establishing the LSP. If any link in that explicit path is down, RSVP signaling fails for that specific path, causing the tunnel to either fall back to dynamic path computation or remain down entirely, rather than using the explicitly configured route. On the Cisco SPCOR 350-501 exam, this scenario tests your understanding of MPLS TE path selection and RSVP signaling behavior—a common trap is to assume a configuration error or metric issue, but the most direct cause is a failed link in the explicit path. A useful memory tip is to remember that explicit paths are strict: if a link is down, the path is dead.
This 350-501 practice question tests your understanding of architecture. 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 service provider is implementing MPLS TE to optimize bandwidth utilization. The engineer notices that tunnels are not using the explicitly configured path. What is the most likely reason?
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.
The explicit path contains a link that is down
Causes path to be invalid, tunnel may use dynamic or stay down.
RSVP is not enabled on the headend
Why wrong: Without RSVP, tunnel would not come up at all.
The tunnel destination is not reachable via IGP
Why wrong: That would prevent tunnel setup regardless of path.
The tunnel is configured with 'autoroute announce'
Why wrong: That affects routing, not path selection.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
The explicit path contains a link that is down
When an MPLS TE tunnel is configured with an explicit path, the headend router uses RSVP to signal the path and verify that all links in the path are operational. If any link in the explicit path is down, RSVP signaling fails for that path, and the tunnel may fall back to dynamic path computation or remain down, rather than using the explicitly configured path. This is the most common reason for a tunnel not using its explicit path.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
The explicit path contains a link that is down
Why this is correct
Causes path to be invalid, tunnel may use dynamic or stay down.
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.
RSVP is not enabled on the headend
Why it's wrong here
Without RSVP, tunnel would not come up at all.
The tunnel destination is not reachable via IGP
Why it's wrong here
That would prevent tunnel setup regardless of path.
The tunnel is configured with 'autoroute announce'
Why it's wrong here
That affects routing, not path selection.
Common exam traps
Cisco often tests the misconception that 'autoroute announce' or IGP reachability issues cause path selection problems, when in fact the explicit path failure is due to a down link in the path itself, which is a fundamental RSVP signaling constraint.
Detailed technical explanation
MPLS TE explicit paths are defined using the 'ip explicit-path' command with a list of next-hop IP addresses or interface identifiers. During RSVP Path message signaling, each hop must be reachable and the corresponding link must be up; if a hop is down, RSVP sends a PathErr message with error code 'Routing Problem' (code 24) and subcode 'Unreachable destination' (subcode 5), causing the tunnel to either use a dynamic path if configured or remain down. In real-world networks, this often occurs during maintenance windows when an interface is shut down but the explicit path is not updated.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
A small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
What to study next
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FAQ
Architecture — This question tests Architecture — Read the scenario before looking for a memorised answer..
The correct answer is: The explicit path contains a link that is down — When an MPLS TE tunnel is configured with an explicit path, the headend router uses RSVP to signal the path and verify that all links in the path are operational. If any link in the explicit path is down, RSVP signaling fails for that path, and the tunnel may fall back to dynamic path computation or remain down, rather than using the explicitly configured path. This is the most common reason for a tunnel not using its explicit path.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 11, 2026
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