350-501 MPLS and Segment Routing Practice Question
A service provider is migrating their MPLS core network to Segment Routing (SR-MPLS). All existing core routers run IOS-XR and are configured with an SRGB of 16000-23999 and OSPF as the IGP. A new router (R5) is added as a PE. The engineer configures R5 with the same SRGB and enables segment-routing under OSPF. However, when configuring an SR-TE policy from R5 to the remote loopback 10.0.0.1 on R1, the policy remains down. Show commands reveal that R5 is not learning the prefix-SID for 10.0.0.1. On R1, the prefix-SID is configured with index 100. The engineer verifies that OSPF adjacencies are up and that R5 can ping 10.0.0.1. What is the most likely cause of the issue?
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
✓
The OSPF process on R5 is not configured with segment-routing mpls.
The most likely cause is that the OSPF process on R5 is not configured with the 'segment-routing mpls' subcommand. Without this configuration, OSPF does not advertise or learn prefix-SIDs, so R5 cannot obtain the prefix-SID for 10.0.0.1. Option B is incorrect: while a color attribute is often used in SR-TE policies, the immediate issue is the lack of prefix-SID learning, not the absence of color. Option C is incorrect: all routers share the same SRGB (16000-23999), so there is no mismatch. Option D is incorrect: index 100 maps to label 16100 (16000+100), which falls within the SRGB range, so this is not the problem.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
The OSPF process on R5 is not configured with segment-routing mpls.
Why this is correct
Without this command, R5 cannot exchange prefix-SID information via OSPF.
- ✗
The SR-TE policy on R5 is missing the color attribute.
Why it's wrong here
While color is important, the policy is defined with a segment-list; the core problem is the missing prefix-SID learning.
- ✗
R1 is not configured with the same SRGB as R5.
Why it's wrong here
Both routers are configured with the same SRGB range.
- ✗
The prefix-SID index 100 is not within the SRGB range of R5 (16000-23999).
Why it's wrong here
Index 100 gives label 16100, which is within the SRGB range.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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