Quick Answer
The correct answer is the Protected Adj-SID. In Segment Routing with TI-LFA, when a link fails, the local repair router must compute a backup path that is guaranteed to be loop-free, and this requires an Adjacency Segment Identifier that has been explicitly flagged as "protected" during label allocation. The protected Adj-SID ensures the backup path steers traffic away from the failed link without risking a routing loop, because it is installed in the forwarding table with a backup NHLFE (Next-Hop Label Forwarding Entry) specifically for fast reroute. On the Cisco SPCOR 350-501 exam, this concept tests your understanding of how TI-LFA leverages the protected flag on Adj-SIDs to achieve sub-50ms convergence, and a common trap is confusing this with an "unprotected" Adj-SID, which lacks any FRR capability. Remember the memory tip: "Protected protects the path; unprotected leaves it unprotected."
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 service provider is using Segment Routing with TI-LFA for fast convergence. During a link failure, the router performing the local repair must compute a backup path that avoids the failed link. Which type of Adjacency Segment Identifier (Adj-SID) is required for the backup path to be loop-free?
Anycast Adj-SID
Why wrong: Anycast Adj-SID is used for load balancing, not local protection.
Unprotected Adj-SID
Why wrong: Unprotected Adj-SIDs are not used for TI-LFA backup.
Protected Adj-SID
Protected Adj-SID enables fast reroute protection in SR networks.
Backup Adj-SID
Why wrong: There is no standard 'Backup Adj-SID'; TI-LFA uses the protected flag.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
Protected Adj-SID
TI-LFA requires the backup path to use a specific Adj-SID that steers traffic away from the failed link. The 'protected' Adj-SID is designed for this purpose. Option A is incorrect because 'unprotected' Adj-SID does not support fast reroute. Option C is incorrect because there is no 'backup' Adj-SID; it's a property. Option D is incorrect because 'anycast' Adj-SID is used for anycast groups, not for FRR.
Key principle: OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
Anycast Adj-SID
Why it's wrong here
Anycast Adj-SID is used for load balancing, not local protection.
Unprotected Adj-SID
Why it's wrong here
Unprotected Adj-SIDs are not used for TI-LFA backup.
Protected Adj-SID
Why this is correct
Protected Adj-SID enables fast reroute protection in SR networks.
Related concept
OSPF neighbours must agree on key parameters.
Backup Adj-SID
Why it's wrong here
There is no standard 'Backup Adj-SID'; TI-LFA uses the protected flag.
Common exam traps
OSPF neighbour formation depends on matching areas, timers, network type, authentication and passive-interface behaviour. Do not choose an answer only because the devices can ping.
Detailed technical explanation
OSPF questions usually test the details that control adjacency and route selection. Read the neighbour state, area, router ID and interface configuration before deciding what is wrong.
Key takeaway
OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.
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
Review OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related 350-501 OSPF questions on adjacency and route selection.
MPLS and Segment Routing — study guide chapter
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FAQ
MPLS and Segment Routing — This question tests MPLS and Segment Routing — OSPF neighbours must agree on key parameters..
The correct answer is: Protected Adj-SID — TI-LFA requires the backup path to use a specific Adj-SID that steers traffic away from the failed link. The 'protected' Adj-SID is designed for this purpose. Option A is incorrect because 'unprotected' Adj-SID does not support fast reroute. Option C is incorrect because there is no 'backup' Adj-SID; it's a property. Option D is incorrect because 'anycast' Adj-SID is used for anycast groups, not for FRR.
Review OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related 350-501 OSPF questions on adjacency and route selection.
OSPF neighbours must agree on key parameters.
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Last reviewed: Jun 24, 2026
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