TI-LFA Protection: Backup Paths and Adjacency-SID Configuration
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?
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.
⚠ Common exam trap
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.
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
✓
Adjacency-SIDs are not configured on neighboring routers
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.
Answer analysis
Option-by-option breakdown
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.
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Same concept, more angles
1 more way this is tested on 350-501
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?
hard- A.All prefixes have backup paths.
- ✓ B.Only 1 prefix has a backup path.
- C.0 prefixes have backup paths.
- D.2 prefixes have backup paths.
Why B: The output shows that for the protected prefix 10.1.1.1/32, the backup path is listed as 'N/A', meaning no backup path was computed. However, for the other prefix (likely a different /32), a backup path is present. This indicates that only one prefix has a backup path, making option B correct. TI-LFA computes a backup path only when a viable post-convergence path exists without microloops; if the topology doesn't allow it, the backup is marked as not available.
JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 350-501 practice question is part of Courseiva's free Cisco certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 350-501 exam.