The correct configuration requires enabling 'segment-routing mpls' globally and then assigning a SID index under the OSPF router process for the specific loopback interface. This is necessary because OSPF must be explicitly instructed to advertise a Prefix-SID; without the global SR-MPLS activation, the OSPF process cannot bind a segment identifier to a prefix, and without the 'prefix-sid index' command under the OSPF router configuration for that loopback network, the router will continue relying on LDP for label distribution. On the Cisco SPCOR 350-501 exam, this scenario tests your understanding of the migration from LDP to Segment Routing, where a common trap is forgetting the global 'segment-routing mpls' command—candidates often only apply the SID index under OSPF, which fails because the SR-MPLS control plane is not enabled. A reliable memory tip is "Global first, then index": think of the global command as flipping the SR switch, and the index command as assigning the seat number for that specific loopback.
350-501 MPLS and Segment Routing Practice Question
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.
During an MPLS network migration from LDP to Segment Routing, an engineer notices that some routers are not advertising Prefix-SIDs for certain loopbacks. The IGP is OSPF. What configuration is required on these routers to advertise Prefix-SIDs?
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' globally and assign a SID index under the OSPF router process for the loopback.
In OSPF, Prefix-SIDs for loopbacks are advertised by configuring 'segment-routing mpls' globally and then assigning a SID index under the OSPF router process using the 'prefix-sid index' command for the specific loopback network. This ties the SID to the OSPF prefix advertisement, enabling SR-MPLS forwarding without LDP.
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 'mpls ldp autoconfig' on the loopback interface.
Why it's wrong here
This is for LDP, not SR.
✗
Enable 'segment-routing mpls' globally and configure 'prefix-sid index' under the loopback interface.
Why it's wrong here
The prefix-sid is configured under the OSPF router process, not the interface.
✗
Configure 'segment-routing mpls set-adjacency-sid' on the loopback.
Why it's wrong here
Adjacency-SIDs are for interfaces, not loopbacks.
✓
Configure 'segment-routing mpls' globally and assign a SID index under the OSPF router process for the loopback.
Why this is correct
This enables SR globally and assigns the Prefix-SID under OSPF.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the distinction between where Prefix-SID configuration is applied (under the IGP process) versus interface-level commands, leading candidates to mistakenly configure 'prefix-sid index' directly on the loopback interface.
Detailed technical explanation
How to think about this question
Segment Routing in OSPF uses the 'prefix-sid' subcommand under the router ospf process to allocate a SID index to a specific prefix (e.g., loopback), which is then advertised in OSPF LSA Type 1 or Type 5 with the SR TLV. The global 'segment-routing mpls' command enables SR-MPLS data plane support, while the OSPF process configuration binds the SID to the prefix. A common real-world scenario is migrating from LDP to SR without changing the IGP topology, where each router's loopback must have a unique SID index to avoid conflicts.
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 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
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.
MPLS and Segment Routing — This question tests MPLS and Segment Routing — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Configure 'segment-routing mpls' globally and assign a SID index under the OSPF router process for the loopback. — In OSPF, Prefix-SIDs for loopbacks are advertised by configuring 'segment-routing mpls' globally and then assigning a SID index under the OSPF router process using the 'prefix-sid index' command for the specific loopback network. This ties the SID to the OSPF prefix advertisement, enabling SR-MPLS forwarding without LDP.
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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