The correct answer is that TI-LFA precomputes a backup path using a segment list (label stack) that avoids the failed link. This mechanism works by calculating a post-convergence path in advance and encoding it as a stack of MPLS segment identifiers (SIDs), which steers traffic around the failure without waiting for routing protocol convergence. On the Cisco SPCOR / CCNP Service Provider Core 350-501 exam, this concept tests your understanding of how Segment Routing leverages label stacking for sub-50ms fast convergence, often contrasting TI-LFA with traditional LFA or RSVP-TE. A common trap is confusing TI-LFA with BFD, which only detects failure, or assuming it uses RSVP-TE signaling—TI-LFA is purely SR-based and pushes SIDs, not RSVP labels. Remember the memory tip: “TI-LFA stacks SIDs to skip the broken link,” emphasizing that it precomputes a segment list for the repair path, not just a next-hop backup.
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 deploying Segment Routing (SR) with TI-LFA for fast convergence. Which mechanism does TI-LFA use to repair a link failure in a node segment path?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Precomputes a backup path using a segment list (label stack) that avoids the failed link
TI-LFA uses a post-convergence path and pushes additional segment IDs (SIDs) for the repair path. Option A is correct because TI-LFA relies on MPLS label stacking to steer traffic around the failure. Option B is wrong because RSVP-TE is different. Option C is wrong because BFD only detects failure. Option D is wrong because LFA alone does not use segment routing.
Key principle: NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✓
Precomputes a backup path using a segment list (label stack) that avoids the failed link
Why this is correct
TI-LFA precomputes a backup path by inserting a segment list to steer traffic around the failure.
Related concept
Static NAT maps one inside address to one outside address.
✗
Uses loop-free alternate (LFA) as defined in IPFRR
Why it's wrong here
LFA may not provide optimal coverage; TI-LFA extends LFA with segment routing.
✗
Uses RSVP-TE to signal a protected LSP
Why it's wrong here
RSVP-TE is not used in SR-TE; TI-LFA is independent of RSVP.
✗
Relies on BFD to trigger a backup path in the forwarding table
Why it's wrong here
BFD aids fast detection but does not compute the backup path.
Common exam traps
Common exam trap: NAT rules depend on direction and matching traffic
NAT is not only about the public address. The inside/outside interface roles and the ACL or rule that matches traffic are just as important.
Detailed technical explanation
How to think about this question
NAT questions usually test address translation, overload/PAT behaviour, static mappings and whether the right traffic is being translated. Read the interface direction and address terms carefully.
KKey Concepts to Remember
Static NAT maps one inside address to one outside address.
PAT allows many inside hosts to share one public address using ports.
Inside local and inside global describe the private and translated addresses.
NAT ACLs identify traffic for translation, not always security filtering.
TExam Day Tips
→Identify inside and outside interfaces first.
→Check whether the scenario needs static NAT, dynamic NAT or PAT.
→Do not confuse NAT matching ACLs with normal packet-filtering intent.
Key takeaway
NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
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.
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related 350-501 NAT questions on configuration and troubleshooting.
Architecture — This question tests Architecture — Static NAT maps one inside address to one outside address..
What is the correct answer to this question?
The correct answer is: Precomputes a backup path using a segment list (label stack) that avoids the failed link — TI-LFA uses a post-convergence path and pushes additional segment IDs (SIDs) for the repair path. Option A is correct because TI-LFA relies on MPLS label stacking to steer traffic around the failure. Option B is wrong because RSVP-TE is different. Option C is wrong because BFD only detects failure. Option D is wrong because LFA alone does not use segment routing.
What should I do if I get this 350-501 question wrong?
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related 350-501 NAT questions on configuration and troubleshooting.
What is the key concept behind this question?
Static NAT maps one inside address to one outside address.
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Question Discussion
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