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 MPLS-TE tunnel is configured with Fast Reroute using link protection. The primary path traverses links A-B and B-C. If link B-C fails, which action does the head-end router take?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "primary"
Why it matters: Asks for the main purpose or function, not a secondary benefit. Eliminate answers that describe side-effects or partial functions.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The router at node B switches traffic to a pre-computed backup tunnel around link B-C.
Option A is correct because MPLS-TE Fast Reroute (FRR) with link protection pre-computes a backup tunnel that bypasses the protected link. When link B-C fails, the router at node B (the Point of Local Repair, or PLR) immediately switches traffic to this pre-established backup tunnel, ensuring sub-50ms failover without involving the head-end router.
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 router at node B switches traffic to a pre-computed backup tunnel around link B-C.
Why this is correct
Link protection works by having the PLR (point of local repair) at the upstream node.
Clue confirmation
The clue word "primary" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
The head-end router immediately switches to a secondary explicit path.
Why it's wrong here
Fast Reroute is local repair; the head-end may not be aware immediately.
✗
RSVP-TE signals a new LSP from the head-end after detecting the failure.
Why it's wrong here
That would be much slower than FRR; FRR uses pre-signaled backup tunnels.
✗
Traffic is dropped until the IGP converges on the new topology.
Why it's wrong here
FRR is designed to provide sub-50ms convergence; no packet loss.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that the head-end router handles all rerouting decisions in MPLS-TE, but FRR delegates local repair to the PLR, so candidates must remember that link protection is handled at the point of failure, not the head-end.
Detailed technical explanation
How to think about this question
In MPLS-TE FRR link protection, the PLR (node B) pre-provisions a backup tunnel that merges back to the original LSP at the Merge Point (MP, node C). The backup tunnel is signaled using RSVP-TE with the 'record route' object and 'fast-reroute' flag, and the PLR uses a label stacking mechanism to switch traffic instantly upon link failure. In real-world deployments, this ensures carrier-grade resilience for voice and video traffic, where even a few hundred milliseconds of loss is unacceptable.
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.
Networking — This question tests Networking — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The router at node B switches traffic to a pre-computed backup tunnel around link B-C. — Option A is correct because MPLS-TE Fast Reroute (FRR) with link protection pre-computes a backup tunnel that bypasses the protected link. When link B-C fails, the router at node B (the Point of Local Repair, or PLR) immediately switches traffic to this pre-established backup tunnel, ensuring sub-50ms failover without involving the head-end router.
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: "primary". Asks for the main purpose or function, not a secondary benefit. Eliminate answers that describe side-effects or partial functions.
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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