The answer is that RSVP-TE is not enabled on the transit interfaces. Even when the link-state database confirms sufficient reservable bandwidth and a dynamic path option is configured, MPLS-TE tunnel establishment fails because RSVP-TE must be explicitly enabled on every interface that will process PATH and RESV messages. Without this, the transit routers cannot participate in label-switched path signaling, so the tunnel remains down despite correct CSPF calculations. On the Cisco SPCOR 350-501 exam, this scenario tests your understanding that MPLS-TE relies on RSVP-TE as the signaling protocol, and a common trap is assuming that a full link-state database guarantees tunnel setup. Remember that reservable bandwidth is necessary but not sufficient—the signaling plane must be active. A useful memory tip is “no RSVP, no tunnel,” meaning every hop in the path needs the `ip rsvp bandwidth` command under the interface.
350-501 Architecture Practice Question
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.
An operator notices that a new MPLS-TE tunnel is not being established. The tunnel configuration includes a dynamic path option and a bandwidth of 100 Mbps. The network uses RSVP-TE with CSPF. The link-state database shows sufficient reservable bandwidth on all links along the calculated path. What is the most likely cause of the tunnel establishment failure?
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
✓
RSVP-TE is not enabled on the transit interfaces
RSVP-TE must be explicitly enabled on every interface that will participate in MPLS-TE label-switched path (LSP) signaling. Even if the link-state database shows sufficient reservable bandwidth, without RSVP-TE enabled on transit interfaces, the PATH messages cannot be processed, and the tunnel will fail to establish. This is a common misconfiguration when deploying MPLS-TE.
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.
✓
RSVP-TE is not enabled on the transit interfaces
Why this is correct
RSVP-TE must be enabled on each interface along the path for signaling.
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 path option is misconfigured with a strict explicit path
Why it's wrong here
The scenario says dynamic path, so explicit path is not used.
✗
MPLS LDP is not enabled on the core routers
Why it's wrong here
LDP is used for label distribution in LDP-based MPLS, but RSVP-TE does not require LDP.
✗
The tunnel source interface is not configured with an IP address
Why it's wrong here
The source interface IP is needed but typically configured; lack of IP would cause a different error.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the distinction between having sufficient bandwidth in the link-state database (IGP) versus having RSVP-TE actually enabled on the interfaces, leading candidates to overlook the required interface-level RSVP configuration.
Trap categories for this question
Scenario analysis trap
The scenario says dynamic path, so explicit path is not used.
Detailed technical explanation
How to think about this question
RSVP-TE uses PATH and RESV messages to reserve bandwidth and distribute labels along the LSP. The 'ip rsvp bandwidth' command must be configured on each interface to enable RSVP processing and allocate reservable bandwidth. Without it, the router silently drops RSVP messages, preventing LSP establishment even if CSPF calculates a valid path.
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 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
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.
Architecture — This question tests Architecture — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: RSVP-TE is not enabled on the transit interfaces — RSVP-TE must be explicitly enabled on every interface that will participate in MPLS-TE label-switched path (LSP) signaling. Even if the link-state database shows sufficient reservable bandwidth, without RSVP-TE enabled on transit interfaces, the PATH messages cannot be processed, and the tunnel will fail to establish. This is a common misconfiguration when deploying MPLS-TE.
What should I do if I get this 350-501 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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Question Discussion
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