The answer is that the path message is rejected due to resource reservation failure. This occurs because, although the CSPF computation identified a path with sufficient bandwidth on paper, the actual RSVP resource reservation process fails when the router attempts to commit the requested 200 Mbps. Even with an established RSVP neighbor, the reservation can be blocked by a mismatch in bandwidth pool configuration—such as using the global pool when the subpool is required—or by transient contention where the exact bandwidth is no longer available at the moment of signaling. On the Cisco SPCOR 350-501 exam, this scenario tests your understanding that CSPF path computation and RSVP resource reservation are distinct phases; a common trap is assuming a successful CSPF run guarantees tunnel activation. Remember: CSPF finds the road, but RSVP must reserve the seat—if the seat is taken or the wrong ticket type is used, the tunnel stays down. A useful mnemonic is “CSPF calculates, RSVP reserves; if the reservation fails, the tunnel never serves.”
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.
In a carrier's network, MPLS-TE tunnels are used to steer traffic away from congested links. The operator configures a tunnel with a bandwidth of 200 Mbps and a dynamic path. The CSPF computation shows a path with sufficient bandwidth, but the tunnel fails to come up. The RSVP neighbor is established. 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
✓
The path message is rejected due to resource reservation failure
The tunnel fails to come up because the CSPF-computed path cannot reserve the requested 200 Mbps bandwidth. Even though the path has sufficient bandwidth on paper, the RSVP resource reservation process may fail due to a lack of available bandwidth at the exact time of reservation, or due to a mismatch in bandwidth pool configuration (e.g., global vs. subpool). Since the RSVP neighbor is established, the issue is specifically with the reservation step, not with reachability or LDP.
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.
✗
The tunnel destination is not reachable via IGP
Why it's wrong here
If unreachable, CSPF would not find a path.
✓
The path message is rejected due to resource reservation failure
Why this is correct
RSVP-TE reserves bandwidth; if not available, tunnel fails.
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 tunnel interface is down
Why it's wrong here
Tunnel interface is a logical interface; it should be up/up if configured correctly.
✗
MPLS LDP is not configured
Why it's wrong here
LDP is not required for RSVP-TE tunnels.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the distinction between CSPF path computation (which only checks link metrics and TE attributes) and RSVP resource reservation (which performs actual bandwidth admission control), leading candidates to incorrectly assume that a successful CSPF computation guarantees the tunnel will come up.
Detailed technical explanation
How to think about this question
In MPLS-TE, RSVP Path messages carry a TSPEC object that requests bandwidth. The downstream router checks the available bandwidth in the configured pool (e.g., global pool or subpool) and, if insufficient, sends a PathErr with 'Admission Control Failure' (error code 01, subcode 02). This can occur even if CSPF initially found a path, because CSPF uses a snapshot of link-state database metrics, while RSVP performs actual admission control at reservation time. Real-world scenarios include oversubscription or misconfigured bandwidth pools (e.g., using 'bc' vs 'be' parameters).
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 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.
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: The path message is rejected due to resource reservation failure — The tunnel fails to come up because the CSPF-computed path cannot reserve the requested 200 Mbps bandwidth. Even though the path has sufficient bandwidth on paper, the RSVP resource reservation process may fail due to a lack of available bandwidth at the exact time of reservation, or due to a mismatch in bandwidth pool configuration (e.g., global vs. subpool). Since the RSVP neighbor is established, the issue is specifically with the reservation step, not with reachability or LDP.
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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