Quick Answer
The answer is that the IGP must be a link-state protocol with complete topology information, such as OSPF or IS-IS. TI-LFA requires this full link-state database because it computes a post-convergence backup path by pre-calculating how traffic would reroute after a failure, which is impossible without a complete map of all routers and links. On the Cisco SPCOR 350-501 exam, this concept tests your understanding that TI-LFA is topology-independent only when the underlying IGP provides the necessary visibility; a common trap is assuming distance-vector protocols like EIGRP suffice, but they lack the detailed link-state database needed for loop-free alternate computation. A strong memory tip is to link “TI-LFA” with “Link-State IGP” by remembering that the “I” in TI-LFA stands for “Independent,” but it is only independent of topology changes, not of the protocol type itself—so always pair it with OSPF or IS-IS.
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 deploying a new MPLS core with Segment Routing and requires fast convergence upon link failure. They plan to use TI-LFA (Topology Independent Loop-Free Alternate). What is a prerequisite for TI-LFA to provide protection against any single link failure?
IGP must be a link-state protocol with complete topology information (OSPF or IS-IS).
TI-LFA uses the link-state database to compute backup paths.
BGP-LU must be enabled for label distribution.
Why wrong: BGP-LU is not required for TI-LFA.
LDP must be enabled on all interfaces.
Why wrong: TI-LFA is independent of LDP.
RSVP-TE must be configured with FRR.
Why wrong: TI-LFA does not use RSVP-TE.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
IGP must be a link-state protocol with complete topology information (OSPF or IS-IS).
TI-LFA relies on the IGP having a complete view of the network topology to compute a post-convergence path that avoids the failed link. OSPF and IS-IS are link-state protocols that flood link-state advertisements (LSAs) or link-state packets (LSPs) to provide this full topology database, which is essential for TI-LFA to calculate a loop-free backup path for any single link failure.
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.
IGP must be a link-state protocol with complete topology information (OSPF or IS-IS).
Why this is correct
TI-LFA uses the link-state database to compute backup paths.
Related concept
Read the scenario before looking for a memorised answer.
BGP-LU must be enabled for label distribution.
Why it's wrong here
BGP-LU is not required for TI-LFA.
LDP must be enabled on all interfaces.
Why it's wrong here
TI-LFA is independent of LDP.
RSVP-TE must be configured with FRR.
Why it's wrong here
TI-LFA does not use RSVP-TE.
Common exam traps
Cisco often tests the misconception that TI-LFA requires LDP or RSVP-TE, but the key prerequisite is a link-state IGP (OSPF or IS-IS) with complete topology information, as TI-LFA is a Segment Routing feature that uses IGP-based SIDs for path computation.
Detailed technical explanation
TI-LFA computes a backup path by identifying a PQ node (a node that is both a P-node and a Q-node) that can provide a loop-free alternate path after the failure. It uses the IGP's link-state database to precompute the post-convergence path and then pushes a segment list (e.g., an adjacency SID) to steer traffic around the failure without waiting for IGP convergence. In real-world deployments, TI-LFA can achieve sub-50ms failover times, similar to RSVP-TE FRR, but without the signaling overhead.
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.
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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: IGP must be a link-state protocol with complete topology information (OSPF or IS-IS). — TI-LFA relies on the IGP having a complete view of the network topology to compute a post-convergence path that avoids the failed link. OSPF and IS-IS are link-state protocols that flood link-state advertisements (LSAs) or link-state packets (LSPs) to provide this full topology database, which is essential for TI-LFA to calculate a loop-free backup path for any single link failure.
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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Last reviewed: Jun 11, 2026
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