The answer is the explicit path with segment list and delay timer. In Segment Routing with TI-LFA, when a link fails, the protecting router computes a backup path encoded as an explicit segment list that steers traffic around the failure, while a delay timer prevents the backup path from activating until the network has fully converged, thereby eliminating transient micro-loops. On the Cisco SPCOR 350-501 exam, this concept tests your understanding of how TI-LFA differs from traditional IP fast reroute by using source-routed segment lists rather than simple next-hop repair paths. A common trap is assuming TI-LFA prevents loops instantly—it does not; the delay timer is critical for convergence. Remember the mnemonic “List then Wait”: the segment list provides the path, and the delay timer provides the wait.
350-501 Services Practice Question
This 350-501 practice question tests your understanding of services. 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 Segment Routing network with TI-LFA enabled, which mechanism prevents micro-loops during a link failure?
Why wrong: Incorrect. CSPF is used for path computation in MPLS-TE, not for loop avoidance.
B
Explicit path with segment list and delay timer
Correct. TI-LFA encodes a post-convergence path as an explicit segment list and introduces a delay to allow other routers to converge, preventing micro-loops.
C
Loop-Free Alternate precomputed backup paths
Why wrong: Incorrect. LFA provides protection but may create micro-loops in certain failure scenarios.
D
Prefix Independent Convergence with fast reroute
Why wrong: Incorrect. PIC provides fast convergence but does not inherently prevent micro-loops.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Explicit path with segment list and delay timer
In Segment Routing with TI-LFA, micro-loops are prevented by using an explicit path with a segment list and a delay timer. When a link fails, the protecting router installs a backup path with a segment list that steers traffic around the failure, and a delay timer ensures that the backup path is not activated until the network has converged, preventing transient loops.
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.
✗
Constraint Shortest Path First computation
Why it's wrong here
Incorrect. CSPF is used for path computation in MPLS-TE, not for loop avoidance.
✓
Explicit path with segment list and delay timer
Why this is correct
Correct. TI-LFA encodes a post-convergence path as an explicit segment list and introduces a delay to allow other routers to converge, preventing micro-loops.
Related concept
Read the scenario before looking for a memorised answer.
✗
Loop-Free Alternate precomputed backup paths
Why it's wrong here
Incorrect. LFA provides protection but may create micro-loops in certain failure scenarios.
✗
Prefix Independent Convergence with fast reroute
Why it's wrong here
Incorrect. PIC provides fast convergence but does not inherently prevent micro-loops.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the distinction between TI-LFA and traditional LFA, where candidates mistakenly think LFA alone prevents micro-loops, but TI-LFA specifically adds segment lists and delay timers to address this issue.
Trap categories for this question
Scenario analysis trap
Incorrect. LFA provides protection but may create micro-loops in certain failure scenarios.
Detailed technical explanation
How to think about this question
TI-LFA (Topology Independent Loop-Free Alternate) computes a backup path using a segment list that encodes the post-convergence path, ensuring that traffic is forwarded along the correct path even before the network fully converges. The delay timer (typically in milliseconds) is configured to allow the IGP to converge before the backup path is used, preventing micro-loops that could occur if the backup path were activated immediately. In real-world scenarios, this is critical for networks with high-speed links where even transient loops can cause packet loss and service disruption.
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.
Services — This question tests Services — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Explicit path with segment list and delay timer — In Segment Routing with TI-LFA, micro-loops are prevented by using an explicit path with a segment list and a delay timer. When a link fails, the protecting router installs a backup path with a segment list that steers traffic around the failure, and a delay timer ensures that the backup path is not activated until the network has converged, preventing transient loops.
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.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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