350-501 MPLS and Segment Routing Practice Question
During an MPLS network migration from LDP to Segment Routing, an engineer notices that some routers are not advertising Prefix-SIDs for certain loopbacks. The IGP is OSPF. What configuration is required on these routers to advertise Prefix-SIDs?
⚠ Common exam trap
Cisco often tests the distinction between where Prefix-SID configuration is applied (under the IGP process) versus interface-level commands, leading candidates to mistakenly configure 'prefix-sid index' directly on the loopback interface.
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
✓
Configure 'segment-routing mpls' globally and assign a SID index under the OSPF router process for the loopback.
In OSPF, Prefix-SIDs for loopbacks are advertised by configuring 'segment-routing mpls' globally and then assigning a SID index under the OSPF router process using the 'prefix-sid index' command for the specific loopback network. This ties the SID to the OSPF prefix advertisement, enabling SR-MPLS forwarding without LDP.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Enable 'mpls ldp autoconfig' on the loopback interface.
Why it's wrong here
This is for LDP, not SR.
- ✗
Enable 'segment-routing mpls' globally and configure 'prefix-sid index' under the loopback interface.
Why it's wrong here
The prefix-sid is configured under the OSPF router process, not the interface.
- ✗
Configure 'segment-routing mpls set-adjacency-sid' on the loopback.
Why it's wrong here
Adjacency-SIDs are for interfaces, not loopbacks.
- ✓
Configure 'segment-routing mpls' globally and assign a SID index under the OSPF router process for the loopback.
Why this is correct
This enables SR globally and assigns the Prefix-SID under OSPF.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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