350-501 Architecture Practice Question
Which TWO are benefits of using Segment Routing over LDP in a service provider core?
⚠ Common exam trap
Cisco often tests the misconception that Segment Routing eliminates IGP convergence entirely, when in fact it still depends on IGP for topology discovery and SID advertisement, but removes the need for a separate label distribution protocol like LDP.
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
✓
Label allocation is per-prefix, not per-interface.
Segment Routing (SR) uses a centralized control plane (via PCE or BGP-LS) to allocate labels per prefix, unlike LDP which allocates labels per interface on a hop-by-hop basis. This reduces the number of labels in the control plane and simplifies label management, as SR-MPLS uses a single label stack entry per segment rather than per-interface labels.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Reduces the number of labels in the control plane.
Why it's wrong here
SR with label stacking can actually increase the number of labels in the data plane.
- ✓
Label allocation is per-prefix, not per-interface.
Why this is correct
SR assigns a single label per prefix independent of the outgoing interface, simplifying label management.
- ✗
Eliminates the need for IGP convergence for label distribution.
Why it's wrong here
SR relies on IGP extensions for label distribution, so IGP convergence still affects label availability.
- ✗
Simplifies MPLS TE tunnel configuration.
Why it's wrong here
SR does not inherently simplify TE; it offers a different approach but LDP also supports TE with RSVP.
- ✓
Supports TI-LFA for fast reroute with full topology protection.
Why this is correct
SR natively enables TI-LFA, providing link and node protection without additional protocols.
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