Quick Answer
The correct answer is the command `fast-reroute per-prefix ti-lfa` under the IGP process. This configuration activates Topology Independent Loop-Free Alternate (TI-LFA) for link protection in an SR network because it instructs IS-IS to compute a backup path for each prefix that avoids the protected link, leveraging segment routing’s source-routing capabilities to guarantee a loop-free alternate regardless of network topology. On the Cisco SPCOR / CCNP Service Provider Core 350-501 exam, this question tests your ability to distinguish TI-LFA from other fast-reroute mechanisms; a common trap is confusing it with MPLS TE auto-tunnel or metric delay commands, which serve entirely different purposes. Remember that TI-LFA is an IGP-level feature, not an MPLS TE or convergence timer tweak. A simple memory tip: “TI-LFA lives under the IGP process, not under MPLS or interface delay.”
This 350-501 practice question tests your understanding of architecture. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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.
In an SR network, which configuration is required to activate TI-LFA for link protection?
metric delay interface configuration
Why wrong: This affects IGP metric propagation, not fast reroute.
label-preferred under the interface
Why wrong: This is not a valid IOS-XE command.
fast-reroute per-prefix ti-lfa under the IGP process
This command enables TI-LFA for all prefixes in the IGP domain.
mpls traffic-eng auto-tunnel primary
Why wrong: This is for auto-generated TE tunnels, not for TI-LFA.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
fast-reroute per-prefix ti-lfa under the IGP process
Option C is correct because the command 'fast-reroute per-prefix ti-lfa' under the IGP process (IS-IS or OSPF) explicitly enables TI-LFA. Option A is wrong because 'mpls traffic-eng auto-tunnel primary' is for TE auto-tunnel, not TI-LFA. Option B is wrong because 'label-preferred' is not a valid command. Option D is wrong because 'metric delay' is for IGP convergence, not TI-LFA.
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.
metric delay interface configuration
Why it's wrong here
This affects IGP metric propagation, not fast reroute.
label-preferred under the interface
Why it's wrong here
This is not a valid IOS-XE command.
fast-reroute per-prefix ti-lfa under the IGP process
Why this is correct
This command enables TI-LFA for all prefixes in the IGP domain.
Related concept
OSPF neighbours must agree on key parameters.
mpls traffic-eng auto-tunnel primary
Why it's wrong here
This is for auto-generated TE tunnels, not for TI-LFA.
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.
Trap categories for this question
Command / output trap
This is not a valid IOS-XE command.
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.
Architecture — study guide chapter
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FAQ
Architecture — This question tests Architecture — OSPF neighbours must agree on key parameters..
The correct answer is: fast-reroute per-prefix ti-lfa under the IGP process — Option C is correct because the command 'fast-reroute per-prefix ti-lfa' under the IGP process (IS-IS or OSPF) explicitly enables TI-LFA. Option A is wrong because 'mpls traffic-eng auto-tunnel primary' is for TE auto-tunnel, not TI-LFA. Option B is wrong because 'label-preferred' is not a valid command. Option D is wrong because 'metric delay' is for IGP convergence, not TI-LFA.
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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