Quick Answer
The answer is Sub-TLV 3, the required IS-IS extension to advertise the prefix-SID for Segment Routing. This sub-TLV is carried within TLV 135 (Extended IP Reachability) and contains the SID value along with flags that control SR behavior, making it the mandatory signaling mechanism for prefix-SIDs in the MPLS core. On the Cisco SPCOR 350-501 exam, this concept tests your understanding of how IS-IS supports Segment Routing by extending existing TLVs rather than introducing new ones—a common trap is confusing Sub-TLV 3 with other sub-TLVs like the Adjacency-SID (Sub-TLV 31). Remember that the prefix-SID is tied to the route itself, so it lives inside the reachability TLV. A useful memory tip: think “3 for the third hop” or simply “Prefix-SID = TLV 135 + Sub-TLV 3.”
This 350-501 practice question tests your understanding of architecture. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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 the MPLS core using IS-IS. Which extension is required in IS-IS to advertise the prefix-SID?
TLV 22 (Extended IS Reachability)
Why wrong: TLV 22 carries adjacency information, not prefix-SID.
TLV 242 (Router Capability)
Why wrong: TLV 242 can carry SIDs but sub-TLV 3 is the actual prefix-SID.
Sub-TLV 3 (Prefix-SID)
Sub-TLV 3 is used within TLV 135 or 242 to carry prefix-SID.
TLV 135 (Extended IP Reachability)
Why wrong: TLV 135 is for extended IP prefixes, not SID.
Answer choices
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.
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.
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.
Related concept
Read the scenario before looking for a memorised answer.
TLV 135 (Extended IP Reachability)
Why it's wrong here
TLV 135 is for extended IP prefixes, not SID.
Common exam traps
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.
Detailed technical explanation
Sub-TLV 3 is defined in RFC 8667 and includes fields for the SID value, flags (such as R-flag for redistribution), and algorithm. In a real-world deployment, misconfiguration of the SRGB or failure to enable the prefix-SID under the IS-IS process can cause the SID to be missing from the LSP, leading to MPLS forwarding failures. The SID is algorithmically derived from the prefix index and SRGB base, but Sub-TLV 3 explicitly carries the final label value.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
A practitioner preparing for the 350-501 exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
What to study next
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FAQ
Architecture — This question tests Architecture — Read the scenario before looking for a memorised answer..
The correct answer is: 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.
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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