This 350-501 practice question tests your understanding of networking. 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 deploying segment routing in its MPLS core. Which label allocation method is used by segment routing to distribute prefix SIDs?
LDP
Why wrong: LDP is a different label distribution protocol, not used in segment routing.
BGP
Why wrong: BGP is used for service labels in MPLS VPN, not for prefix SIDs.
RSVP-TE
Why wrong: RSVP-TE is used for traffic engineering, not for prefix SID distribution.
IGP (IS-IS or OSPF)
Segment routing encodes prefix SIDs in IGP updates.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
IGP (IS-IS or OSPF)
Segment routing uses the IGP (IS-IS or OSPF) to distribute prefix SIDs. The IGP extensions for segment routing (RFC 8665 for OSPF, RFC 8667 for IS-IS) carry the prefix SID sub-TLV within the prefix reachability information, allowing each router to allocate and advertise the SID associated with a prefix. This is the native label allocation method for segment routing, as it leverages the existing IGP database without requiring a separate label distribution protocol.
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.
LDP
Why it's wrong here
LDP is a different label distribution protocol, not used in segment routing.
BGP
RSVP-TE
Why it's wrong here
RSVP-TE is used for traffic engineering, not for prefix SID distribution.
IGP (IS-IS or OSPF)
Why this is correct
Segment routing encodes prefix SIDs in IGP updates.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Cisco often tests the misconception that segment routing requires a separate label distribution protocol like LDP or RSVP-TE, but the trap here is that segment routing uses the IGP itself to distribute prefix SIDs, making it a control-plane-less label distribution method.
Detailed technical explanation
Under the hood, the IGP floods the prefix SID as a 32-bit value (for IPv4) or 128-bit value (for IPv6) within the router's link-state advertisements. The SID is globally unique within the segment routing domain (SRGB) and is used to program the forwarding table with an MPLS label. In a real-world scenario, if an operator migrates from LDP to segment routing, they must ensure the IGP is configured with the segment-routing mpls command and that the SRGB range does not overlap with existing LDP labels to avoid forwarding loops during migration.
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.
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FAQ
Networking — This question tests Networking — Read the scenario before looking for a memorised answer..
The correct answer is: IGP (IS-IS or OSPF) — Segment routing uses the IGP (IS-IS or OSPF) to distribute prefix SIDs. The IGP extensions for segment routing (RFC 8665 for OSPF, RFC 8667 for IS-IS) carry the prefix SID sub-TLV within the prefix reachability information, allowing each router to allocate and advertise the SID associated with a prefix. This is the native label allocation method for segment routing, as it leverages the existing IGP database without requiring a separate label distribution protocol.
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 11, 2026
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