350-501 Prefix-SID Practice Question
In Segment Routing, what is the role of the 'Prefix-SID'?
⚠ Common exam trap
The common misconception is confusing Prefix-SID with Adjacency-SID. A Prefix-SID is bound to a prefix (e.g., a loopback) and leads to forwarding along the shortest path to that prefix, while an Adjacency-SID is bound to a specific adjacency and forces traffic over that link. Remember that Prefix-SID identifies a prefix in the IGP, not an adjacency or a service chain.
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
✓
It identifies a prefix in the IGP
A Prefix-SID is a Segment Routing identifier bound to an IGP prefix. It allows forwarding along the shortest path computed by the IGP to that prefix. Therefore, it identifies a prefix in the IGP, not a specific adjacency or node.
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
This describes an Adjacency-SID, not a Prefix-SID.
- ✗
It is used for service chaining
Why it's wrong here
Service chaining uses service SIDs or other constructs, not Prefix-SID.
- ✗
It identifies a specific node
Why it's wrong here
A Prefix-SID is bound to a prefix, not a node. While a node may have a unique loopback prefix, the SID represents the prefix.
- ✓
It identifies a prefix in the IGP
Why this is correct
Correct. A Prefix-SID identifies an IGP prefix and is used to route traffic to that prefix via the shortest path.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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