Quick Answer
The answer is the missing 'segment-routing mpls' command under the IS-IS process. Without this explicit command, IS-IS cannot allocate MPLS labels for prefixes using the Segment Routing extension, so the router defaults to LDP for label distribution, which explains why some prefixes still receive LDP labels during a Segment Routing migration from LDP. On the Cisco SPCOR 350-501 exam, this tests your understanding that enabling segment-routing under IS-IS alone is insufficient—you must also enable the MPLS data plane integration. A common trap is assuming that enabling segment-routing globally or under the interface automatically triggers label allocation, but IS-IS requires the explicit 'segment-routing mpls' subcommand to bind SR labels to prefixes. Remember the mnemonic: "SR needs MPLS under IS-IS to flip the label switch."
This 350-501 practice question tests your understanding of mpls and segment routing. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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 in an IS-IS network. After enabling IS-IS with segment-routing on all routers, they observe that some prefixes still receive labels from LDP instead of from SR. Which configuration is most likely missing on these routers?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Missing 'segment-routing mpls' command under IS-IS
This command enables SR label allocation in IS-IS.
OSPF segment-routing is still configured
Why wrong: OSPF is not involved if the IGP is IS-IS.
TI-LFA is not enabled
Why wrong: TI-LFA is unrelated to basic label allocation.
SRGB range conflicts with MPLS label range
Why wrong: Conflict would cause error, but not just reliance on LDP.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
Missing 'segment-routing mpls' command under IS-IS
When migrating from LDP to Segment Routing in an IS-IS network, the 'segment-routing mpls' command must be explicitly enabled under the IS-IS routing process. Without this command, IS-IS will not allocate MPLS labels for prefixes using the Segment Routing (SR) extension, causing the router to fall back to LDP for label distribution. This is the most common missing configuration when SR labels are not being assigned.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
Missing 'segment-routing mpls' command under IS-IS
Why this is correct
This command enables SR label allocation in IS-IS.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
OSPF segment-routing is still configured
Why it's wrong here
OSPF is not involved if the IGP is IS-IS.
TI-LFA is not enabled
Why it's wrong here
TI-LFA is unrelated to basic label allocation.
SRGB range conflicts with MPLS label range
Why it's wrong here
Conflict would cause error, but not just reliance on LDP.
Common exam traps
Cisco often tests the distinction between enabling segment-routing globally versus enabling it under the IGP process; the trap here is that candidates assume 'segment-routing mpls' is automatically applied when SR is enabled globally, but it must be explicitly configured under IS-IS or OSPF.
Detailed technical explanation
Under the hood, the 'segment-routing mpls' command under IS-IS triggers the allocation of MPLS labels from the SRGB (Segment Routing Global Block) for each prefix-SID. Without this command, IS-IS will not advertise prefix-SIDs or install MPLS labels, leaving LDP as the default label distribution protocol. In real-world migrations, engineers often forget this step because they enable segment-routing globally but omit the per-protocol activation, causing a hybrid state where some routers use SR and others rely on LDP.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
A network engineer at a university connects two campus buildings via a fibre link. Both routers run OSPF, but no adjacency forms — even though both routers can ping each other. The engineer finds one router is in area 0 and the other in area 1. OSPF adjacency requires matching area numbers, hello/dead timers, and network type. IP reachability alone is not enough.
What to study next
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MPLS and Segment Routing — study guide chapter
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FAQ
MPLS and Segment Routing — This question tests MPLS and Segment Routing — Read the scenario before looking for a memorised answer..
The correct answer is: Missing 'segment-routing mpls' command under IS-IS — When migrating from LDP to Segment Routing in an IS-IS network, the 'segment-routing mpls' command must be explicitly enabled under the IS-IS routing process. Without this command, IS-IS will not allocate MPLS labels for prefixes using the Segment Routing (SR) extension, causing the router to fall back to LDP for label distribution. This is the most common missing configuration when SR labels are not being assigned.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 25, 2026
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