- A
Configure TI-LFA on all interfaces using 'fast-reroute per-prefix ti-lfa interface'
Why wrong: Interface-level TI-LFA only protects against link failures, not node failures; the issue is node protection.
- B
Add the 'sr-protect' option under the fast-reroute configuration to enable node protection
The 'sr-protect' option ensures that the node acts as a protecting node for transit traffic, providing node protection even if the node itself is the failure point.
- C
Reduce the IGP timers and enable incremental SPF to accelerate convergence
Why wrong: Reducing timers may help but does not provide the sub-50ms protection that TI-LFA is designed for; the issue is that TI-LFA did not compute a backup path.
- D
Deploy LDP as a fallback label distribution protocol to provide additional backup paths
Why wrong: LDP does not provide fast reroute guarantees and would not achieve sub-50ms convergence.
Quick Answer
The answer is to add the 'sr-protect' option under the fast-reroute configuration to enable node protection. This is correct because TI-LFA, by default, often provides only link protection, meaning it computes a backup path that avoids the failed link but may still traverse the failed node. When a core router like P1 fails and the BGP next-hop is multiple hops away, the backup path can still be impacted if the protecting node does not explicitly compute a path that bypasses the failed node entirely. The 'sr-protect' keyword forces the router to compute a node-protecting backup path that avoids the failing router itself, ensuring sub-50ms convergence even for transit traffic. On the Cisco SPCOR 350-501 exam, this scenario tests your understanding of segment routing TI-LFA protection levels and the distinction between link and node protection. A common trap is assuming that enabling TI-LFA automatically provides node protection, but the 'sr-protect' option is required for that guarantee. Memory tip: think of "sr-protect" as "segment routing protects the node, not just the link."
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.
A large service provider operates a national MPLS backbone with over 200 P routers and 500 PE routers. They use IS-IS as the IGP with segment routing and have deployed TI-LFA for link and node protection. Recently, a core router (P1) suffered a complete failure, and during the failure, traffic for some prefixes was dropped for over 200ms. After the failure, the network recovered within seconds. The engineer suspects that TI-LFA did not provide the expected sub-50ms protection for some destinations. Further analysis reveals that the affected prefixes have their BGP next-hop on a router that is multiple hops away, and the P1 failure impacted both the primary path and the backup path computed by TI-LFA. The engineer reviews the TI-LFA configuration and finds that 'fast-reroute per-prefix ti-lfa' is enabled under router isis. The engineer also notes that P1 was not a protecting node for those prefixes. Which action should the engineer take to improve convergence time for these prefixes?
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 choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
Add the 'sr-protect' option under the fast-reroute configuration to enable node protection
Option B is correct because for TI-LFA to provide node protection, the 'sr-protect' option should be configured, which forces the node to attempt to provide protection even for traffic that transits through the node. Option A is wrong because interface-level TI-LFA would not help; the issue is node-level. Option C is wrong because increasing IGP timers may cause slower convergence. Option D is wrong because there is no need for an additional LDP backup.
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.
- ✗
Configure TI-LFA on all interfaces using 'fast-reroute per-prefix ti-lfa interface'
Why it's wrong here
Interface-level TI-LFA only protects against link failures, not node failures; the issue is node protection.
- ✓
Add the 'sr-protect' option under the fast-reroute configuration to enable node protection
Why this is correct
The 'sr-protect' option ensures that the node acts as a protecting node for transit traffic, providing node protection even if the node itself is the failure point.
Clue confirmation
The clue word "primary" in the question point toward this answer.
Related concept
Static NAT maps one inside address to one outside address.
- ✗
Reduce the IGP timers and enable incremental SPF to accelerate convergence
Why it's wrong here
Reducing timers may help but does not provide the sub-50ms protection that TI-LFA is designed for; the issue is that TI-LFA did not compute a backup path.
- ✗
Deploy LDP as a fallback label distribution protocol to provide additional backup paths
Why it's wrong here
LDP does not provide fast reroute guarantees and would not achieve sub-50ms convergence.
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.
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FAQ
Questions learners often ask
What does this 350-501 question test?
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: Add the 'sr-protect' option under the fast-reroute configuration to enable node protection — Option B is correct because for TI-LFA to provide node protection, the 'sr-protect' option should be configured, which forces the node to attempt to provide protection even for traffic that transits through the node. Option A is wrong because interface-level TI-LFA would not help; the issue is node-level. Option C is wrong because increasing IGP timers may cause slower convergence. Option D is wrong because there is no need for an additional LDP backup.
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.
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?
Static NAT maps one inside address to one outside address.
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Last reviewed: Jun 24, 2026
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