350-501 MPLS and Segment Routing Practice Question
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?
⚠ Common exam trap
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.
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
✓
The 'prefix-sid index' command is not associated with the loopback prefix; it should be configured under the OSPF process for that specific prefix.
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.
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'.
- ✗
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.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 350-501 practice question is part of Courseiva's free Cisco certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 350-501 exam.