350-501 Networking Practice Question
A service provider is deploying segment routing in its MPLS core. Which label allocation method is used by segment routing to distribute prefix SIDs?
⚠ Common exam trap
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.
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
✓
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.
Answer analysis
Option-by-option breakdown
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
Why it's wrong here
BGP is used for service labels in MPLS VPN, not for prefix SIDs.
- ✗
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.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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