350-501 MPLS and Segment Routing Practice Question
A network engineer is designing an MPLS core using Segment Routing. They want to minimize label stack depth while still providing fast convergence using TI-LFA. Which prefix-SID assignment strategy should be used?
⚠ Common exam trap
Cisco often tests the misconception that node-SIDs (loopbacks) are sufficient for TI-LFA, but the trap is that TI-LFA requires prefix-SIDs for all prefixes to achieve optimal label stack depth and fast convergence, as node-SIDs alone may force deeper stacks or suboptimal backup paths.
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
✓
Advertise prefix-SIDs for all IGP prefixes
Advertising prefix-SIDs for all IGP prefixes allows TI-LFA to compute backup paths using any prefix in the network, not just loopbacks. This minimizes label stack depth by enabling TI-LFA to use a single prefix-SID for the repair path, avoiding the need for additional labels or explicit path constructs. Fast convergence is achieved because TI-LFA precomputes a backup next-hop that can be activated immediately upon failure, and having all prefixes as SIDs ensures the backup path can be expressed with minimal label imposition.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Assign a node-SID to each loopback interface
Why it's wrong here
Node-SID is typical, but the question is about minimizing label stack while using TI-LFA.
- ✗
Use per-interface label mode from SRGB
Why it's wrong here
Interface-based labels increase stack depth.
- ✗
Allocate labels per-VRF on PE routers
Why it's wrong here
VRF labels are for VPN, not transport.
- ✓
Advertise prefix-SIDs for all IGP prefixes
Why this is correct
Prefix-SIDs enable TI-LFA with minimal label depth.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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