350-501 Architecture Practice Question
A service provider is deploying Segment Routing in the MPLS core using IS-IS. Which extension is required in IS-IS to advertise the prefix-SID?
⚠ Common exam trap
Cisco often tests the distinction between the TLV that carries the prefix (TLV 135) and the sub-TLV that carries the SID (Sub-TLV 3), leading candidates to incorrectly select TLV 135 as the answer.
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
✓
Sub-TLV 3 (Prefix-SID)
In IS-IS, the Prefix-SID is advertised using Sub-TLV 3, which is carried within TLV 135 (Extended IP Reachability). This sub-TLV contains the SID value and flags, enabling Segment Routing in the MPLS core. Without Sub-TLV 3, the prefix-SID cannot be signaled, making it the required extension.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
TLV 22 (Extended IS Reachability)
Why it's wrong here
TLV 22 carries adjacency information, not prefix-SID.
- ✗
TLV 242 (Router Capability)
Why it's wrong here
TLV 242 can carry SIDs but sub-TLV 3 is the actual prefix-SID.
- ✓
Sub-TLV 3 (Prefix-SID)
Why this is correct
Sub-TLV 3 is used within TLV 135 or 242 to carry prefix-SID.
- ✗
TLV 135 (Extended IP Reachability)
Why it's wrong here
TLV 135 is for extended IP prefixes, not SID.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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