350-501 MPLS and Segment Routing Practice Question
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?
⚠ Common exam trap
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.
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
✓
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.
Answer analysis
Option-by-option breakdown
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.
- ✗
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.
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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