The answer is that external routes are not covered by prefix-SIDs, which is why they are missing labels in an IS-IS Segment Routing migration. This occurs because Segment Routing with IS-IS only assigns prefix-SIDs to prefixes that are part of the IS-IS domain and explicitly configured with a SID; external routes redistributed from protocols like BGP or OSPF fall outside this scope and require a separate label distribution mechanism such as LDP or a manually configured explicit-null label to be forwarded with MPLS encapsulation. On the Cisco SPCOR / CCNP Service Provider Core 350-501 exam, this concept tests your understanding of the boundary between SR-native prefixes and redistributed routes—a common trap is assuming SR automatically labels all routes in the routing table. Remember the memory tip: “SR SIDs are for IS-IS insiders only; outsiders need LDP.”
350-501 MPLS and Segment Routing Practice Question
This 350-501 practice question tests your understanding of mpls and segment routing. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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 network operator is migrating from traditional MPLS LDP to Segment Routing (SR) with IS-IS. The network consists of four routers (R1-R4) in a square topology. The engineer has enabled SR on all routers and configured prefix-SIDs for loopbacks. However, when checking the MPLS forwarding table on R1, the engineer sees that some prefixes have label values that are not the prefix-SIDs. For example, the prefix for R4's loopback shows label 16004 instead of the expected 16004 (which is correct). But for another prefix, the label is 16003 instead of 16003 (correct). The engineer does not see any labels for some external routes. What is the most likely reason that some labels are missing?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
External routes are not covered by prefix-SIDs; they require LDP or another label distribution mechanism.
Option D is correct because Segment Routing (SR) with IS-IS only assigns prefix-SIDs to prefixes that are part of the IS-IS domain and explicitly configured with a SID. External routes, such as those redistributed from another protocol (e.g., BGP or OSPF), are not covered by prefix-SIDs and require a separate label distribution mechanism like LDP or a manually configured explicit-null label to be forwarded with MPLS encapsulation. The engineer's observation that some labels are missing for external routes directly points to this limitation.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
The IS-IS wide metrics are not enabled on all interfaces.
Why it's wrong here
Wide metrics are needed for SR but not the cause of missing labels.
✗
Route redistribution from another protocol is not configured.
Why it's wrong here
Redistribution does not automatically assign labels.
✗
The prefix-SIDs are inconsistent across routers.
Why it's wrong here
The engineer saw correct SIDs for some prefixes.
✓
External routes are not covered by prefix-SIDs; they require LDP or another label distribution mechanism.
Why this is correct
SR only assigns labels to IGP prefixes; external routes need separate handling.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that SR with IS-IS automatically assigns labels to all routes in the routing table, when in fact prefix-SIDs only apply to IGP routes within the same protocol domain, and external routes still need LDP or another label distribution mechanism.
Detailed technical explanation
How to think about this question
In SR-MPLS, prefix-SIDs are allocated from the Segment Routing Global Block (SRGB) and advertised via IS-IS TLV 135 (Extended IP Reachability) with sub-TLV 3 (Prefix-SID). External routes learned via redistribution are not part of the IS-IS link-state database and thus cannot carry a prefix-SID; they require either LDP to bind labels or a manual label-switched path (LSP) via explicit configuration. A common real-world scenario is when an operator migrates from LDP to SR but forgets to keep LDP enabled for redistributed routes, causing black-holing of traffic to external destinations.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
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.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
MPLS and Segment Routing — This question tests MPLS and Segment Routing — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: External routes are not covered by prefix-SIDs; they require LDP or another label distribution mechanism. — Option D is correct because Segment Routing (SR) with IS-IS only assigns prefix-SIDs to prefixes that are part of the IS-IS domain and explicitly configured with a SID. External routes, such as those redistributed from another protocol (e.g., BGP or OSPF), are not covered by prefix-SIDs and require a separate label distribution mechanism like LDP or a manually configured explicit-null label to be forwarded with MPLS encapsulation. The engineer's observation that some labels are missing for external routes directly points to this limitation.
What should I do if I get this 350-501 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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