Quick Answer
The correct choice is to advertise prefix-SIDs for all IGP prefixes. This strategy works because TI-LFA computes a backup path using any prefix in the network, not just loopbacks, which allows the repair path to be expressed with a single prefix-SID instead of requiring additional labels or explicit path constructs. By having all prefixes as SIDs, the label stack depth is minimized while still achieving fast convergence through precomputed backup next-hops that activate immediately upon failure. On the Cisco SPCOR 350-501 exam, this question tests your understanding of how TI-LFA interacts with Segment Routing label stack minimization—a common trap is assuming only loopback prefixes are needed, but that forces deeper stacks. Remember the memory tip: "All prefixes, one SID—TI-LFA’s minimal stack is a done deal."
This 350-501 practice question tests your understanding of mpls and segment routing. 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 network engineer is designing an MPLS core using Segment Routing. They want to minimize label stack depth while still providing fast convergence using TI-LFA. Which prefix-SID assignment strategy should be used?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "minimum / minimize"
Why it matters: Asks for the least resource use — fewest addresses, smallest subnet, lowest overhead. Eliminate over-provisioned options even if they would technically work.
Assign a node-SID to each loopback interface
Why wrong: Node-SID is typical, but the question is about minimizing label stack while using TI-LFA.
Use per-interface label mode from SRGB
Why wrong: Interface-based labels increase stack depth.
Allocate labels per-VRF on PE routers
Why wrong: VRF labels are for VPN, not transport.
Advertise prefix-SIDs for all IGP prefixes
Prefix-SIDs enable TI-LFA with minimal label depth.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
Advertise prefix-SIDs for all IGP prefixes
Option D is correct because advertising prefix-SIDs for all IGP prefixes allows TI-LFA to compute backup paths using any prefix in the network, not just loopbacks. This minimizes label stack depth by enabling TI-LFA to use a single prefix-SID for the repair path, avoiding the need for additional labels or explicit path constructs. Fast convergence is achieved because TI-LFA precomputes a backup next-hop that can be activated immediately upon failure, and having all prefixes as SIDs ensures the backup path can be expressed with minimal label imposition.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
Assign a node-SID to each loopback interface
Why it's wrong here
Node-SID is typical, but the question is about minimizing label stack while using TI-LFA.
Use per-interface label mode from SRGB
Why it's wrong here
Interface-based labels increase stack depth.
Allocate labels per-VRF on PE routers
Why it's wrong here
VRF labels are for VPN, not transport.
Advertise prefix-SIDs for all IGP prefixes
Why this is correct
Prefix-SIDs enable TI-LFA with minimal label depth.
Clue confirmation
The clue word "minimum / minimize" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Cisco often tests the misconception that node-SIDs (loopbacks) are sufficient for TI-LFA, but the trap is that TI-LFA requires prefix-SIDs for all prefixes to achieve optimal label stack depth and fast convergence, as node-SIDs alone may force deeper stacks or suboptimal backup paths.
Detailed technical explanation
TI-LFA uses a post-convergence path computed from the IGP link-state database, and it encodes the repair path as a list of segment IDs (SIDs). When all IGP prefixes have prefix-SIDs, the repair list can often be a single SID (the destination prefix-SID) plus an adjacency-SID for the outgoing interface, minimizing the stack. In contrast, if only node-SIDs exist, TI-LFA may need to insert multiple node-SIDs to steer traffic around the failure, increasing the label stack. This is particularly important in large-scale networks where label imposition hardware limits exist, such as on older ASICs that support only a few labels.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
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
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FAQ
MPLS and Segment Routing — This question tests MPLS and Segment Routing — Read the scenario before looking for a memorised answer..
The correct answer is: Advertise prefix-SIDs for all IGP prefixes — Option D is correct because advertising prefix-SIDs for all IGP prefixes allows TI-LFA to compute backup paths using any prefix in the network, not just loopbacks. This minimizes label stack depth by enabling TI-LFA to use a single prefix-SID for the repair path, avoiding the need for additional labels or explicit path constructs. Fast convergence is achieved because TI-LFA precomputes a backup next-hop that can be activated immediately upon failure, and having all prefixes as SIDs ensures the backup path can be expressed with minimal label imposition.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Yes — watch for: "minimum / minimize". Asks for the least resource use — fewest addresses, smallest subnet, lowest overhead. Eliminate over-provisioned options even if they would technically work.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 25, 2026
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