350-501 SR-LDP interworking Practice Question
A service provider is migrating from LDP to Segment Routing in its MPLS core. The team has enabled IS-IS as the IGP and configured segment routing under the IS-IS process on all core routers. However, after the migration, some LSPs are not being signaled correctly and traffic is blackholing. Which action should be taken to ensure seamless interworking between LDP and SR during the migration?
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
✓
Configure 'segment-routing mpls sr-prefer' under the IS-IS process.
The 'segment-routing mpls sr-prefer' command under IS-IS configures the router to prefer SR labels for forwarding while maintaining LDP forwarding entries for fallback. This ensures seamless interworking between LDP and SR during migration, preventing blackholing when some routers are not fully SR-capable. Option B is incorrect because changing the IGP to OSPF is unnecessary and disruptive; IS-IS can support both LDP and SR simultaneously. Option C is incorrect because removing LDP from all routers would break LDP signaling for any router not yet migrated to SR, causing traffic loss. Option D is incorrect because 'mpls ldp igp sync' synchronizes LDP with IGP convergence, not SR-LDP interworking, and does not address label preference during coexistence.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Configure 'segment-routing mpls sr-prefer' under the IS-IS process.
Why this is correct
Correct. The 'segment-routing mpls sr-prefer' command ensures SR labels are preferred but LDP labels are maintained for fallback, enabling seamless coexistence during migration.
- ✗
Change the IGP to OSPF with segment routing enabled.
Why it's wrong here
Incorrect. Changing the IGP to OSPF is unnecessary and would cause disruption; IS-IS supports both LDP and SR simultaneously.
- ✗
Remove the 'mpls ldp' configuration from all routers that have SR enabled.
Why it's wrong here
Incorrect. Removing LDP from all routers would break LDP signaling for routers not yet migrated to SR, causing traffic blackholing.
- ✗
Enable 'mpls ldp igp sync' on the IGP interfaces.
Why it's wrong here
Incorrect. 'mpls ldp igp sync' synchronizes LDP with IGP convergence and does not handle SR-LDP label preference; it is not the solution for interworking.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 350-501 practice question is part of Courseiva's free Cisco certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 350-501 exam.