The correct technique is to use a BGP-free core with LDP or Segment Routing. This design works because edge routers (PEs) impose a two-label stack: an outer IGP label distributed by LDP or Segment Routing for the BGP next-hop, and an inner BGP label for the prefix. Core (P) routers only need to swap the outer label based on the IGP label, never inspecting the BGP prefix, which eliminates the need for an IGP label per BGP prefix. On the Cisco SPCOR / CCNP Service Provider Core 350-501 exam, this concept tests your understanding of label imposition and forwarding hierarchy—a common trap is assuming P routers must run BGP or maintain per-prefix labels. The key insight is that the BGP-free core offloads BGP processing to the edge, keeping the core simple and scalable. Memory tip: think of the outer label as the “envelope” for the next-hop, and the inner BGP label as the “letter” for the prefix—the core only reads the envelope.
350-501 MPLS and Segment Routing Practice Question
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.
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?
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.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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.
Related concept
Read the scenario before looking for a memorised answer.
✗
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.
Common exam traps
Common exam trap: answer the scenario, not the keyword
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.
Detailed technical explanation
How to think about this question
In a BGP-free core, the core routers (P routers) only run an IGP (e.g., OSPF or IS-IS) and LDP or Segment Routing, and they do not run BGP. When a labeled packet arrives, the P router performs a label swap on the outer label (IGP label) to forward the packet toward the BGP next-hop (PE router). The inner BGP label is only examined by the egress PE router, which pops it to determine the destination. This design reduces the control-plane load on core routers and is commonly used in large-scale MPLS networks, as specified in RFC 3031 and RFC 3107.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
MPLS and Segment Routing — This question tests MPLS and Segment Routing — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: 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.
What should I do if I get this 350-501 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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