350-501 MPLS and Segment Routing Practice Question
A service provider is implementing MPLS in their core network. They want to ensure that labeled packets for BGP prefixes are forwarded correctly without requiring an IGP label for every BGP prefix. Which technique should be used to achieve this?
⚠ Common exam trap
Cisco often tests the misconception that LDP can distribute labels for BGP prefixes, but LDP is strictly for IGP prefixes; the correct approach is to use a BGP-free core where LDP or Segment Routing handles the IGP label for the BGP next-hop, not the BGP prefix itself.
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
✓
Use BGP-free core with LDP or Segment Routing
A BGP-free core with LDP or Segment Routing allows the service provider to forward labeled packets for BGP prefixes without requiring an IGP label for every BGP prefix. In this design, edge routers (PEs) impose a label stack where the outer label (IGP label) is distributed by LDP or Segment Routing for the BGP next-hop, and the inner label is the BGP label. Core routers (P routers) only need to swap the outer label based on the IGP label, not the BGP prefix, thus eliminating the need for IGP labels per BGP prefix.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Use BGP-free core with LDP or Segment Routing
Why this is correct
BGP-free core allows forwarding based on labels assigned by LDP/SR for BGP next hops.
- ✗
Enable MPLS on all interfaces and rely on BGP label distribution
Why it's wrong here
BGP can distribute labels, but this is not the standard design for BGP-free core.
- ✗
Implement LDP for BGP prefixes
Why it's wrong here
LDP assigns labels to IGP prefixes, not BGP prefixes directly.
- ✗
Configure static labels for BGP prefixes
Why it's wrong here
Static labels are not scalable and not recommended for large networks.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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