Quick Answer
The answer is the Prefix-SID and the Adjacency-SID, as these are the two primary types of SR SIDs defined in Segment Routing. A Prefix-SID is a label associated with an IGP prefix, such as a loopback interface, and can be either global or local, enabling traffic to follow the shortest path toward that destination. In contrast, an Adjacency-SID is a local label tied to a specific link between two routers, forcing traffic out that exact interface regardless of the IGP metric. On the Cisco SPCOR 350-501 exam, this distinction tests your understanding of how SR builds forwarding paths—Prefix-SIDs for node reachability and Adjacency-SIDs for strict link steering. A common trap is confusing the Node-SID (a type of Prefix-SID) with the Adjacency-SID, or assuming that only global SIDs exist. To remember, think of the Prefix-SID as the "where to go" (destination) and the Adjacency-SID as the "which door to leave through" (exact interface).
This 350-501 practice question tests your understanding of mpls and segment routing. 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.
Which TWO types of SIDs are defined in Segment Routing? (Select two.)
Adjacency-SID
Identifies a link adjacency.
TI-LFA SID
Why wrong: TI-LFA is fast reroute, not a SID type.
Service-SID
Why wrong: Not a standard SR SID type.
Prefix-SID
Identifies an IP prefix.
Node-SID
Why wrong: Node-SID is a subset of prefix-SID for the router's loopback.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
Adjacency-SID
Segment Routing (SR) defines two primary types of Segment Identifiers (SIDs): the Prefix-SID and the Adjacency-SID. A Prefix-SID is a global or local label associated with an IGP prefix (e.g., a loopback), enabling shortest-path forwarding toward that prefix. An Adjacency-SID is a local label associated with a specific adjacency (link) between two routers, forcing traffic out that exact interface regardless of the IGP shortest path.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
Adjacency-SID
Why this is correct
Identifies a link adjacency.
Related concept
Read the scenario before looking for a memorised answer.
TI-LFA SID
Why it's wrong here
TI-LFA is fast reroute, not a SID type.
Service-SID
Why it's wrong here
Not a standard SR SID type.
Prefix-SID
Why this is correct
Identifies an IP prefix.
Related concept
Read the scenario before looking for a memorised answer.
Node-SID
Why it's wrong here
Node-SID is a subset of prefix-SID for the router's loopback.
Common exam traps
Cisco often tests the distinction between Node-SID and Prefix-SID, where candidates mistakenly treat Node-SID as a separate SID type, but it is actually a Prefix-SID assigned to the router's loopback address.
Detailed technical explanation
In SR-MPLS, the Adjacency-SID is allocated from the dynamic label range (e.g., 16–1048575) and is advertised only to neighbors via IGP extensions (IS-IS or OSPF). The Prefix-SID can be either global (SRGB-based) or local; when global, it is unique across the SR domain and enables ECMP load-balancing. A common real-world scenario is using an Adjacency-SID to steer traffic over a specific link for traffic engineering, bypassing the IGP shortest path.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
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
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MPLS and Segment Routing — study guide chapter
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FAQ
MPLS and Segment Routing — This question tests MPLS and Segment Routing — Read the scenario before looking for a memorised answer..
The correct answer is: Adjacency-SID — Segment Routing (SR) defines two primary types of Segment Identifiers (SIDs): the Prefix-SID and the Adjacency-SID. A Prefix-SID is a global or local label associated with an IGP prefix (e.g., a loopback), enabling shortest-path forwarding toward that prefix. An Adjacency-SID is a local label associated with a specific adjacency (link) between two routers, forcing traffic out that exact interface regardless of the IGP shortest path.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 25, 2026
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