350-501 MPLS and Segment Routing Practice Question
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?
⚠ Common exam trap
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.
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
✓
External routes are not covered by prefix-SIDs; they require LDP or another label distribution mechanism.
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.
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.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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