This 350-501 practice question tests your understanding of services. 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 migrating from LDP to Segment Routing. What is the correct order of operations to ensure uninterrupted MPLS forwarding?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Enable SR on each router one by one while LDP remains active, then remove LDP after SR is stable.
Option A is correct because it follows the recommended migration strategy of running LDP and Segment Routing (SR) in parallel. By enabling SR on each router one by one while LDP remains active, MPLS forwarding continues uninterrupted via LDP until SR is fully deployed and stable. Once SR is verified on all routers, LDP can be safely removed without causing any forwarding black holes.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✓
Enable SR on each router one by one while LDP remains active, then remove LDP after SR is stable.
Why this is correct
Correct. This gradual migration ensures continuous forwarding via LDP while SR is established.
Related concept
Read the scenario before looking for a memorised answer.
✗
Enable SR on all routers simultaneously.
Why it's wrong here
Incorrect. This can cause signaling mismatches and loss of connectivity.
✗
Configure IS-IS SR extensions on all routers, then disable LDP.
Why it's wrong here
Incorrect. Disabling LDP before SR is fully deployed will break MPLS forwarding.
✗
Disable LDP first to free labels, then enable SR.
Why it's wrong here
Incorrect. Disabling LDP first will remove all LDP labels, causing a forwarding outage.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that you must disable the old protocol before enabling the new one, but the correct approach is to run both protocols in parallel to maintain forwarding continuity during migration.
Detailed technical explanation
How to think about this question
During migration, LDP and SR can coexist using the 'label mode per-prefix' or 'label mode per-vrf' commands to control which protocol allocates labels for which prefixes. The router uses the LDP label for forwarding until an SR label is installed, ensuring no packet loss. In real-world deployments, operators often use a phased approach, enabling SR on a few routers, verifying IGP-SR adjacency SIDs, and then gradually removing LDP neighbors.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A practitioner preparing for the 350-501 exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Services — This question tests Services — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Enable SR on each router one by one while LDP remains active, then remove LDP after SR is stable. — Option A is correct because it follows the recommended migration strategy of running LDP and Segment Routing (SR) in parallel. By enabling SR on each router one by one while LDP remains active, MPLS forwarding continues uninterrupted via LDP until SR is fully deployed and stable. Once SR is verified on all routers, LDP can be safely removed without causing any forwarding black holes.
What should I do if I get this 350-501 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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