350-501 MPLS and Segment Routing Practice Question
A service provider is transitioning from LDP-based MPLS to Segment Routing (SR-MPLS). Which TWO statements correctly describe differences or interoperability considerations? (Choose two.)
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
✓
The Segment Routing Global Block (SRGB) is a reserved label range that is used to allocate Prefix-SIDs, ensuring end-to-end label consistency.
In SR-MPLS, the Segment Routing Global Block (SRGB) is a reserved label range used to allocate Prefix-SIDs, ensuring end-to-end label consistency across the SR domain. An Adjacency-SID is a local label that represents a specific link and is used for traffic steering over that link. LDP and SR-MPLS can coexist with proper mapping, making option B incorrect. Node-SIDs are allocated from the SRGB and are unique domain-wide, not just within an area. In SR-MPLS, the ingress router pushes a label stack, and intermediate nodes perform penultimate hop popping (PHP) or continue to forward, but they do not swap labels as in LDP. Therefore, options C and D are correct.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
In SR-MPLS, the Node-SID is allocated from the dynamic label range and is unique only within the IGP area.
Why it's wrong here
Incorrect. Node-SID is allocated from SRGB and must be unique globally (domain-wide).
- ✗
LDP and SR-MPLS cannot coexist on the same router; the router must be configured exclusively for one or the other.
Why it's wrong here
Incorrect. They can coexist with mapping if needed.
- ✓
The Segment Routing Global Block (SRGB) is a reserved label range that is used to allocate Prefix-SIDs, ensuring end-to-end label consistency.
Why this is correct
Correct. SRGB is a global label range for Prefix-SIDs.
- ✓
Adjacency-SID in SR-MPLS is a local label representing a specific link and is used for traffic steering over that link.
Why this is correct
Correct. Adjacency-SID is locally significant for a specific adjacency.
- ✗
In SR-MPLS, the label stack is built by the ingress router and nodes swap the top label without pushing additional labels.
Why it's wrong here
Incorrect. In SR-MPLS, the ingress pushes a label stack; intermediate nodes may swap (or pop) labels based on the SID type.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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