The answer is that the 'prefix-sid index' command was not associated with the loopback prefix under the OSPF process. In Cisco IOS XR, a Prefix-SID is a label index bound to a specific prefix within the link-state database, not an interface property; configuring it under the loopback interface alone does not inject the SID into OSPF’s opaque LSA, so the router will not advertise the Prefix-SID. This question tests your understanding of Segment Routing configuration hierarchy on the Cisco SPCOR / CCNP Service Provider Core 350-501 exam, where a common trap is confusing interface-level commands with protocol-level prefix mapping. The correct placement is under 'router ospf 1' with a route policy or direct 'prefix-sid index' for the exact prefix, not under the interface. Memory tip: think "OSPF owns the SID, not the interface"—the index must live inside the routing process, not on the port.
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.
Exhibit
Refer to the exhibit.
interface GigabitEthernet0/0
ip address 10.1.1.1 255.255.255.252
mpls ip
segment-routing mpls
!
router ospf 1
segment-routing mpls
prefix-sid index 100
network 10.0.0.0 0.255.255.255 area 0
!
interface Loopback0
ip address 192.168.1.1 255.255.255.255
ip ospf 1 area 0
!
Refer to the exhibit. An engineer configured Segment Routing on a router. However, the router does not advertise a Prefix-SID for the loopback0 interface. What is the most likely reason?
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.
Refer to the exhibit.
interface GigabitEthernet0/0
ip address 10.1.1.1 255.255.255.252
mpls ip
segment-routing mpls
!
router ospf 1
segment-routing mpls
prefix-sid index 100
network 10.0.0.0 0.255.255.255 area 0
!
interface Loopback0
ip address 192.168.1.1 255.255.255.255
ip ospf 1 area 0
!
A
The loopback is not included in any OSPF network statement.
Why wrong: The loopback is enabled under OSPF via 'ip ospf 1 area 0'.
B
The 'prefix-sid index' command is not associated with the loopback prefix; it should be configured under the OSPF process for that specific prefix.
The prefix-sid index under OSPF is generic; it needs to be tied to the specific prefix, e.g., 'prefix-sid index 100 192.168.1.1/32'.
C
The 'segment-routing mpls' command is missing under the loopback interface.
Why wrong: The loopback does not need 'segment-routing mpls' under it; SR is enabled globally and under the OSPF process.
D
The 'mpls ip' command is missing on the loopback interface.
Why wrong: 'mpls ip' is not required on loopback for SR; it's for LDP.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The 'prefix-sid index' command is not associated with the loopback prefix; it should be configured under the OSPF process for that specific prefix.
The correct answer is B because in Cisco IOS XR, the 'prefix-sid index' command must be configured under the OSPF process for the specific prefix (e.g., 'prefix-sid index 100' under 'router ospf 1' for the loopback0 prefix). Configuring it under the interface does not associate the SID with the prefix in OSPF's link-state database, so the router will not advertise the Prefix-SID. The exhibit shows the command under the loopback interface, which is incorrect for OSPF Segment Routing.
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 loopback is not included in any OSPF network statement.
Why it's wrong here
The loopback is enabled under OSPF via 'ip ospf 1 area 0'.
✓
The 'prefix-sid index' command is not associated with the loopback prefix; it should be configured under the OSPF process for that specific prefix.
Why this is correct
The prefix-sid index under OSPF is generic; it needs to be tied to the specific prefix, e.g., 'prefix-sid index 100 192.168.1.1/32'.
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.
✗
The 'segment-routing mpls' command is missing under the loopback interface.
Why it's wrong here
The loopback does not need 'segment-routing mpls' under it; SR is enabled globally and under the OSPF process.
✗
The 'mpls ip' command is missing on the loopback interface.
Why it's wrong here
'mpls ip' is not required on loopback for SR; it's for LDP.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the distinction between interface-level and protocol-level configuration for Segment Routing, trapping candidates who assume the 'prefix-sid index' command belongs on the interface (like MPLS or IPv6 commands) rather than under the OSPF or IS-IS process.
Detailed technical explanation
How to think about this question
In OSPF Segment Routing, the Prefix-SID is advertised as a Type 1 or Type 3 LSA with a SID sub-TLV, which is generated only when the 'prefix-sid index' is configured under the OSPF process for that prefix. The index value maps to an MPLS label (16000 + index by default) and is distributed via OSPF extensions (RFC 8665). A common real-world scenario is misconfiguring the SID under the interface instead of the OSPF process, leading to no Prefix-SID advertisement and broken traffic engineering paths.
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: The 'prefix-sid index' command is not associated with the loopback prefix; it should be configured under the OSPF process for that specific prefix. — The correct answer is B because in Cisco IOS XR, the 'prefix-sid index' command must be configured under the OSPF process for the specific prefix (e.g., 'prefix-sid index 100' under 'router ospf 1' for the loopback0 prefix). Configuring it under the interface does not associate the SID with the prefix in OSPF's link-state database, so the router will not advertise the Prefix-SID. The exhibit shows the command under the loopback interface, which is incorrect for OSPF Segment Routing.
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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