The answer is that one-to-one backup creates a unique backup tunnel for each protected LSP, which directly causes the label increase. In MPLS-TE FRR, one-to-one backup allocates a dedicated set of labels for every primary LSP that requires protection, meaning each protected path gets its own distinct backup label stack. This contrasts with facility backup, where a single bypass tunnel is shared among many LSPs, conserving labels. On the Cisco SPCOR 350-501 exam, this question tests your understanding of the trade-off between label efficiency and per-LSP granularity; a common trap is confusing one-to-one with facility backup’s shared model. Remember the memory tip: “One-to-one = One backup per LSP, so labels multiply; facility = One bypass for many, so labels stay few.”
350-501 Architecture Practice Question
This 350-501 practice question tests your understanding of architecture. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. 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 with FRR (Fast Reroute) using one-to-one backup tunnels. After activation, they notice that the number of labels in the forwarding table has increased significantly. What is the most likely reason for this increase?
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
✓
One-to-one backup creates a unique backup tunnel for each protected LSP
One-to-one backup tunnels in MPLS-TE FRR create a dedicated backup LSP for each protected primary LSP. This means every primary LSP gets its own unique set of labels for the backup path, leading to a significant increase in the number of labels in the forwarding table. In contrast, facility backup shares a single bypass tunnel among multiple protected LSPs, which conserves labels.
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.
✓
One-to-one backup creates a unique backup tunnel for each protected LSP
Why this is correct
Each protected LSP gets its own backup tunnel, increasing label count linearly.
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-TE signaling is not properly aggregating labels
Why it's wrong here
RSVP-TE signaling does not cause label increase; it's the backup method.
✗
Penultimate Hop Popping (PHP) is disabled
Why it's wrong here
PHP affects label removal, not number of labels in the forwarding table.
✗
Facility backup is being used instead of one-to-one
Common exam trap: answer the scenario, not the keyword
Cisco often tests the distinction between one-to-one and facility backup, where candidates may confuse the label increase with a signaling or PHP issue, rather than recognizing it as a direct consequence of the one-to-one backup design.
Detailed technical explanation
How to think about this question
In one-to-one backup, each protected LSP requires a separate backup tunnel with its own label allocation, so if you have 100 primary LSPs, you may have 100 additional backup labels. This is defined in RFC 4090, which specifies that one-to-one backup creates a distinct detour LSP for each protected LSP. In a large-scale network, this can lead to label exhaustion or increased memory usage on routers, making facility backup more scalable for many LSPs.
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: One-to-one backup creates a unique backup tunnel for each protected LSP — One-to-one backup tunnels in MPLS-TE FRR create a dedicated backup LSP for each protected primary LSP. This means every primary LSP gets its own unique set of labels for the backup path, leading to a significant increase in the number of labels in the forwarding table. In contrast, facility backup shares a single bypass tunnel among multiple protected LSPs, which conserves labels.
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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