- A
The OSPF protocol cannot be used for SR-MPLS because it does not support SR extensions.
Why wrong: Incorrect. OSPF has SR extensions (OSPFv2 SR) and can be used for SR-MPLS.
- B
Adjacency SIDs are local to a router and indicate a specific link.
Correct. Adjacency SIDs are locally significant and represent a specific interface or link.
- C
A prefix SID is a global label that identifies a specific prefix in the network.
Correct. Prefix SIDs are global and identify a prefix or node.
- D
The SRGB must be identical across all routers in the domain to ensure global uniqueness.
Why wrong: Incorrect. While recommended, the SRGB can differ; prefix SIDs must be globally unique but the block itself can vary.
- E
Segment IDs (SIDs) are allocated from the SRGB (Segment Routing Global Block) and advertised via IS-IS.
Correct. SIDs are allocated from SRGB and distributed via IS-IS.
Quick Answer
The correct answer identifies that Segment IDs (SIDs) are allocated from the SRGB and advertised via IS-IS, because in SR-MPLS architecture, the SRGB defines the global label range from which prefix SIDs are drawn, while adjacency SIDs remain locally significant to each router. This distinction is central to understanding how SR-MPLS steers traffic: a prefix SID acts as a global label for a specific network prefix, enabling end-to-end path selection, whereas an adjacency SID is a local label that forces traffic out a specific interface. On the Cisco SPCOR 350-501 exam, this topic tests your grasp of SR-MPLS control-plane behavior, often appearing in questions that ask you to differentiate between SID types or to identify correct statements about SRGB consistency—a common trap is assuming the SRGB must be identical on every router, but it only needs to be overlapping, not uniform. A helpful memory tip: think of prefix SIDs as “global GPS coordinates” for destinations, and adjacency SIDs as “local turn-by-turn directions” for immediate next hops.
350-501 Architecture Practice Question
This 350-501 practice question tests your understanding of architecture. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. 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.
An SP is migrating its core network to Segment Routing (SR-MPLS). The network uses IS-IS as the IGP with SR extensions. Which three statements about SR-MPLS architecture are correct?
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
Adjacency SIDs are local to a router and indicate a specific link.
Segment IDs (SIDs) are allocated from the SRGB and advertised via IS-IS; a prefix SID is a global label identifying a specific prefix; adjacency SIDs are local to a router. The SRGB does not have to be identical across all routers, and OSPF also supports SR.
Key principle: OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.
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 protocol cannot be used for SR-MPLS because it does not support SR extensions.
Why it's wrong here
Incorrect. OSPF has SR extensions (OSPFv2 SR) and can be used for SR-MPLS.
- ✓
Adjacency SIDs are local to a router and indicate a specific link.
Why this is correct
Correct. Adjacency SIDs are locally significant and represent a specific interface or link.
Related concept
OSPF neighbours must agree on key parameters.
- ✓
A prefix SID is a global label that identifies a specific prefix in the network.
Why this is correct
Correct. Prefix SIDs are global and identify a prefix or node.
Related concept
OSPF neighbours must agree on key parameters.
- ✗
The SRGB must be identical across all routers in the domain to ensure global uniqueness.
Why it's wrong here
Incorrect. While recommended, the SRGB can differ; prefix SIDs must be globally unique but the block itself can vary.
- ✓
Segment IDs (SIDs) are allocated from the SRGB (Segment Routing Global Block) and advertised via IS-IS.
Why this is correct
Correct. SIDs are allocated from SRGB and distributed via IS-IS.
Related concept
OSPF neighbours must agree on key parameters.
Common exam traps
Common exam trap: OSPF can fail even when IP connectivity looks correct
OSPF neighbour formation depends on matching areas, timers, network type, authentication and passive-interface behaviour. Do not choose an answer only because the devices can ping.
Detailed technical explanation
How to think about this question
OSPF questions usually test the details that control adjacency and route selection. Read the neighbour state, area, router ID and interface configuration before deciding what is wrong.
KKey Concepts to Remember
- OSPF neighbours must agree on key parameters.
- Router ID selection can affect neighbour relationships and LSDB output.
- OSPF cost influences the preferred path.
- A route can appear in OSPF information but not become the installed route.
TExam Day Tips
- Check area mismatch first when OSPF adjacency fails.
- Review passive interfaces when a network is advertised but no neighbour forms.
- Use show ip ospf neighbor and show ip route clues carefully.
Key takeaway
OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.
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.
Review OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related 350-501 OSPF questions on adjacency and route selection.
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FAQ
Questions learners often ask
What does this 350-501 question test?
Architecture — This question tests Architecture — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: Adjacency SIDs are local to a router and indicate a specific link. — Segment IDs (SIDs) are allocated from the SRGB and advertised via IS-IS; a prefix SID is a global label identifying a specific prefix; adjacency SIDs are local to a router. The SRGB does not have to be identical across all routers, and OSPF also supports SR.
What should I do if I get this 350-501 question wrong?
Review OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related 350-501 OSPF questions on adjacency and route selection.
What is the key concept behind this question?
OSPF neighbours must agree on key parameters.
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Last reviewed: Jun 24, 2026
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