The answer is that the `segment-routing mpls` command is missing under the IS-IS process. This is because, in IS-IS Segment Routing, adjacency SIDs are not automatically allocated simply by enabling SR globally; the IGP process itself must be explicitly configured to support MPLS Segment Routing. Without this subcommand under the `router isis` configuration, the router will not advertise ADJ-SIDs on any interface, even if prefix-SIDs are correctly assigned to loopbacks. On the Cisco SPCOR 350-501 exam, this is a classic trick—candidates often remember to enable `segment-routing mpls` globally but forget the per-process activation, which is the specific step that triggers adjacency SID advertisement. A reliable memory tip is to think of it as a two-layer key: global SR turns on the car, but the IS-IS process command puts it in drive.
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 service provider is migrating from LDP-based MPLS to Segment Routing (SR-MPLS) with IS-IS. After enabling SR on all routers, the adjacency segment IDs (ADJ-SIDs) are not being advertised. Which configuration is missing?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The `segment-routing mpls` command is not enabled under the IS-IS process
In IS-IS, adjacency segment IDs are programmed only when a specific prefix-SID (e.g., the loopback) is configured and SR is enabled globally. Without an SR-capable IGP process on the interface, ADJ-SIDs are not allocated. The most common missing piece is the `segment-routing mpls` command under the IS-IS process.
Key principle: OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.
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 `segment-routing mpls` command is not enabled under the IS-IS process
Why this is correct
IS-IS requires the segment-routing mpls command under the IS-IS process to advertise SR capabilities.
The question states IS-IS is used, so that's not the issue.
✗
The interface has the `isis adjacency-sid` command incorrectly configured
Why it's wrong here
ADJ-SIDs are automatically generated; explicit configuration is not required.
✗
The loopback interface does not have an ip address configured
Why it's wrong here
Loopback is needed for prefix-SID but not for ADJ-SID advertisement.
Common exam traps
Common exam trap: OSPF can fail even when IP connectivity looks correct
OSPF neighbour formation depends on matching areas, timers, network type, authentication and passive-interface behaviour. Do not choose an answer only because the devices can ping.
Detailed technical explanation
How to think about this question
OSPF questions usually test the details that control adjacency and route selection. Read the neighbour state, area, router ID and interface configuration before deciding what is wrong.
KKey Concepts to Remember
OSPF neighbours must agree on key parameters.
Router ID selection can affect neighbour relationships and LSDB output.
OSPF cost influences the preferred path.
A route can appear in OSPF information but not become the installed route.
TExam Day Tips
→Check area mismatch first when OSPF adjacency fails.
→Review passive interfaces when a network is advertised but no neighbour forms.
→Use show ip ospf neighbor and show ip route clues carefully.
Key takeaway
OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.
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.
Review OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related 350-501 OSPF questions on adjacency and route selection.
MPLS and Segment Routing — This question tests MPLS and Segment Routing — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: The `segment-routing mpls` command is not enabled under the IS-IS process — In IS-IS, adjacency segment IDs are programmed only when a specific prefix-SID (e.g., the loopback) is configured and SR is enabled globally. Without an SR-capable IGP process on the interface, ADJ-SIDs are not allocated. The most common missing piece is the `segment-routing mpls` command under the IS-IS process.
What should I do if I get this 350-501 question wrong?
Review OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related 350-501 OSPF questions on adjacency and route selection.
What is the key concept behind this question?
OSPF neighbours must agree on key parameters.
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Question Discussion
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