Quick Answer
The answer is that adjacency-SIDs are not configured on neighboring routers. This is the correct choice because TI-LFA protection for per-prefix fast-reroute in OSPF relies on adjacency-SIDs to compute loop-free backup paths; without these labels on neighboring routers, OSPF cannot build the necessary repair tunnels, leaving the prefix unprotected and showing “No FRR backup” in the output of show ip ospf segment-routing protected. On the Cisco SPCOR 350-501 exam, this scenario tests your understanding that TI-LFA is topology-independent only when adjacency-SIDs are present—a common trap is assuming per-prefix FRR alone suffices, but the adjacency-SID is the critical enabler for backup path computation. Remember the mnemonic: “No Adj-SID, No Backup” to recall that TI-LFA protection requires adjacency-SIDs on all neighboring routers for complete coverage.
This 350-501 practice question tests your understanding of mpls and segment routing. 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 network operator is deploying Segment Routing with TI-LFA across an OSPF network. After configuring OSPF with 'segment-routing mpls' and 'fast-reroute per-prefix', they notice that backup paths are not being installed for some prefixes. 'Show ip ospf segment-routing protected' shows 'No FRR backup' for those prefixes. What is a possible reason?
LDP label distribution is still active
Why wrong: LDP is not required for TI-LFA in SR.
The prefix-SID index is not globally unique
Why wrong: Global uniqueness affects label assignment, not backup path computation.
OSPF does not support TI-LFA for per-prefix prefixes
Why wrong: OSPF does support TI-LFA.
Adjacency-SIDs are not configured on neighboring routers
TI-LFA uses adjacency-SIDs to create the repair path.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
Adjacency-SIDs are not configured on neighboring routers
Option D is correct because TI-LFA (Topology Independent Loop-Free Alternate) for per-prefix fast-reroute in OSPF requires that adjacency-SIDs be configured on neighboring routers. Without adjacency-SIDs, OSPF cannot compute the necessary repair paths to provide backup protection, resulting in 'No FRR backup' for those prefixes.
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.
LDP label distribution is still active
Why it's wrong here
LDP is not required for TI-LFA in SR.
The prefix-SID index is not globally unique
Why it's wrong here
Global uniqueness affects label assignment, not backup path computation.
OSPF does not support TI-LFA for per-prefix prefixes
Why it's wrong here
OSPF does support TI-LFA.
Adjacency-SIDs are not configured on neighboring routers
Why this is correct
TI-LFA uses adjacency-SIDs to create the repair path.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Cisco often tests the misconception that TI-LFA works automatically with just 'fast-reroute per-prefix', when in fact adjacency-SIDs are a prerequisite for the repair path computation in OSPF.
Detailed technical explanation
TI-LFA relies on the post-convergence path computed using the IGP's SPF algorithm and requires adjacency-SIDs to steer traffic along the repair path. Without adjacency-SIDs on neighboring routers, OSPF cannot encode the necessary label stack to bypass the failed link or node, leaving the prefix unprotected. In real-world deployments, this often occurs when operators enable 'fast-reroute per-prefix' but forget to configure 'segment-routing mpls' on all interfaces or omit the adjacency-SID assignment.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
A network engineer at a university connects two campus buildings via a fibre link. Both routers run OSPF, but no adjacency forms — even though both routers can ping each other. The engineer finds one router is in area 0 and the other in area 1. OSPF adjacency requires matching area numbers, hello/dead timers, and network type. IP reachability alone is not enough.
What to study next
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MPLS and Segment Routing — study guide chapter
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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: Adjacency-SIDs are not configured on neighboring routers — Option D is correct because TI-LFA (Topology Independent Loop-Free Alternate) for per-prefix fast-reroute in OSPF requires that adjacency-SIDs be configured on neighboring routers. Without adjacency-SIDs, OSPF cannot compute the necessary repair paths to provide backup protection, resulting in 'No FRR backup' for those prefixes.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Read the scenario before looking for a memorised answer.
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Same concept, more angles
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. What does the output indicate about the TI-LFA protection on R1?
hardWhy B: The output shows that only one prefix has a backup path. Node protection is enabled, and SRLG protection is disabled.
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Last reviewed: Jun 25, 2026
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