The correct answer is to configure 'segment-routing mpls' globally and assign a prefix-SID to the loopback interface under OSPF. This works because SR-MPLS with OSPF requires the IGP to distribute label information via prefix-SIDs, replacing the need for LDP; the global command enables the segment routing control plane, while the 'prefix-sid' command under the OSPF process attaches a specific label index to the loopback, which OSPF then advertises in its Link-State Advertisements using the OSPF extensions for segment routing. On the Cisco SPCOR 350-501 exam, this question tests your understanding of the fundamental SR-MPLS deployment prerequisite—many candidates mistakenly enable SR only under OSPF or forget the global command, so remember that SR must be turned on at the router level first. A common trap is thinking OSPF alone can advertise SIDs without the global 'segment-routing mpls' activation. Memory tip: think "Global first, then OSPF prefix-SID" to avoid the trap of enabling SR only under the routing protocol.
350-501 Networking Practice Question
This 350-501 practice question tests your understanding of networking. 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 deploying segment routing (SR) with MPLS data plane. The network uses OSPF as the IGP. Which configuration is required to enable SR-MPLS and ensure that routers advertise prefix-SIDs for their loopback interfaces?
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 prefix-SID to the loopback interface under OSPF.
Option D is correct because to enable SR-MPLS with OSPF, you must globally enable segment routing with the 'segment-routing mpls' command, and then assign a prefix-SID to the loopback interface under the OSPF process using the 'prefix-sid' command. This ensures that routers advertise the prefix-SID for their loopback via OSPF extensions, which is the fundamental requirement for SR-MPLS operation 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.
✗
Configure 'mpls ip' on all interfaces and 'router ospf 1 mpls ldp auto-config'.
Why it's wrong here
This enables LDP, not segment routing.
✗
Configure 'label mode per-prefix' under the OSPF process and assign a label to the loopback.
Why it's wrong here
This is LDP label allocation, not SR.
✗
Configure 'segment-routing global-block 16000 23999' globally and enable 'mpls ip' on all interfaces.
Why it's wrong here
The global-block defines the SRGB but does not enable SR; 'mpls ip' enables LDP, not SR.
✓
Configure 'segment-routing mpls' globally and assign a prefix-SID to the loopback interface under OSPF.
Why this is correct
This enables SR-MPLS and advertises a prefix-SID for the loopback.
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 enabling segment routing globally versus configuring the SRGB; candidates mistakenly think that setting the SRGB alone enables SR-MPLS, but the 'segment-routing mpls' command is the actual enabler.
Detailed technical explanation
How to think about this question
Under the hood, SR-MPLS with OSPF uses OSPF extensions defined in RFC 8665 to advertise prefix-SIDs as Type-1 or Type-7 opaque LSAs. The SRGB defines the label range (e.g., 16000-23999) from which prefix-SIDs are allocated, and the actual label for a prefix is computed as SRGB base + SID index. In a real-world scenario, this eliminates the need for LDP and reduces control-plane complexity, making it ideal for large-scale service provider networks.
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.
Networking — This question tests Networking — 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 prefix-SID to the loopback interface under OSPF. — Option D is correct because to enable SR-MPLS with OSPF, you must globally enable segment routing with the 'segment-routing mpls' command, and then assign a prefix-SID to the loopback interface under the OSPF process using the 'prefix-sid' command. This ensures that routers advertise the prefix-SID for their loopback via OSPF extensions, which is the fundamental requirement for SR-MPLS operation 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.
About these practice questions
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These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. An engineer is deploying Segment Routing in an MPLS network. To ensure that routers can forward packets based on SR-MPLS labels without requiring LDP, which requirement must be met?
hard
✓ A.IS-IS or OSPF must have the SR extension enabled.
B.MPLS LDP must be enabled on all interfaces.
C.All routers must run BGP-LU.
D.The IGP must have the overload bit set.
Why A: Option C is correct because for SR-MPLS forwarding, the IGP (IS-IS or OSPF) must have the Segment Routing extension enabled to distribute prefix-SIDs. Option A is incorrect; BGP-LU is not required for SR-MPLS. Option B is incorrect; LDP is not needed if SR is used. Option D is incorrect; the overload bit is unrelated.
Last reviewed: Jun 11, 2026
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