The answer is that the Prefix-SID identifies a prefix in the IGP. This is correct because the Prefix-SID is a globally unique segment identifier assigned to a specific prefix, such as a loopback interface, enabling shortest path forwarding based on the IGP’s topology. In the context of the Cisco SPCOR 350-501 exam, this concept tests your understanding of how Segment Routing leverages IGP protocols like OSPF or IS-IS to steer traffic without relying on MPLS labels for every hop. A common trap is confusing the Prefix-SID with the Adjacency-SID, which identifies a specific link rather than a network prefix, or mixing it up with service chaining SIDs or VPN labels. To avoid this, remember the memory tip: “Prefix points to the path, Adjacency points to the port.”
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.
In Segment Routing, what is the role of the 'Prefix-SID'?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
It identifies a prefix in the IGP
The Prefix-SID is globally unique and identifies a prefix in the IGP. It is used for shortest path forwarding. Adjacency-SID identifies a link, service chaining uses other SIDs, and VPN label is separate.
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.
✗
It identifies a specific adjacency
Why it's wrong here
Adjacency-SID identifies an adjacency, not Prefix-SID.
✗
It is used for service chaining
Why it's wrong here
Service chaining may use service SIDs, not Prefix-SID.
✗
It identifies a prefix in the IGP
Why it's wrong here
Correction: Prefix-SID does identify a prefix, but I reordered options. Actually the correct is now D.
✓
It identifies a prefix in the IGP
Why this is correct
Prefix-SID is assigned to a prefix and is globally unique.
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.
Architecture — This question tests Architecture — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: It identifies a prefix in the IGP — The Prefix-SID is globally unique and identifies a prefix in the IGP. It is used for shortest path forwarding. Adjacency-SID identifies a link, service chaining uses other SIDs, and VPN label is separate.
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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Question Discussion
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