350-501 MPLS and Segment Routing Practice Question
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?
⚠ Common exam trap
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.
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
✓
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.
Answer analysis
Option-by-option breakdown
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.
- ✗
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.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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