350-501 MPLS and Segment Routing Practice Question
In Segment Routing over MPLS (SR-MPLS), how are Prefix-SIDs advertised in IS-IS?
⚠ Common exam trap
Cisco often tests the distinction between traditional MPLS label distribution protocols (LDP, RSVP-TE) and SR-MPLS's use of IGP extensions (IS-IS TLVs or OSPF Opaque LSAs) to carry segment IDs, leading candidates to mistakenly associate Prefix-SIDs with LDP or RSVP-TE.
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
✓
Via IS-IS TLV 135 and TLV 149
In SR-MPLS, Prefix-SIDs are advertised in IS-IS using TLV 135 (Extended IP Reachability) to carry the prefix and its metric, and TLV 149 (IS-IS Router Capability) to carry the SID/Label Binding sub-TLV, which maps the prefix to a specific label (Prefix-SID). This allows IS-IS to distribute both the prefix reachability and its associated segment identifier in a single protocol update, enabling source routing without LDP or RSVP-TE.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Via LDP label mapping messages
Why it's wrong here
LDP is separate from SR; SR uses IGP extensions.
- ✗
Via RSVP-TE PATH messages
Why it's wrong here
RSVP-TE is for traffic engineering, not SID distribution.
- ✗
Via BGP VPN labels
Why it's wrong here
BGP is for inter-domain.
- ✓
Via IS-IS TLV 135 and TLV 149
Why this is correct
Correct. IS-IS uses these TLVs for SR SID advertisement.
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