The correct action is to configure the 'segment-routing mpls sr-prefer' command under the IS-IS process. This command ensures seamless interworking between LDP and Segment Routing during migration by instructing the router to prefer SR labels for forwarding while still maintaining LDP label bindings as a fallback, preventing traffic blackholing when some LSPs are not signaled correctly. On the Cisco SPCOR 350-501 exam, this scenario tests your understanding of MPLS control plane coexistence—specifically, that both protocols must signal the same FEC to avoid forwarding gaps. A common trap is to assume removing LDP globally or enabling LDP-IGP synchronization will solve the issue, but those actions either break connectivity for non-SR routers or address a different synchronization problem. Remember the memory tip: "SR-prefer keeps the LDP net, so traffic won't forget."
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 to Segment Routing in its MPLS core. The team has enabled IS-IS as the IGP and configured segment routing under the IS-IS process on all core routers. However, after the migration, some LSPs are not being signaled correctly and traffic is blackholing. Which action should be taken to ensure seamless interworking between LDP and SR during the migration?
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 sr-prefer' under the IS-IS process.
Option C is correct because during migration, both LDP and SR need to signal the same labels. The 'segment-routing mpls sr-prefer' command tells the router to prefer SR labels but still maintain LDP forwarding entries for fallback. Option A is incorrect because removing LDP globally would cause loss of LDP sessions for routers not yet SR-capable. Option B is incorrect because 'mpls ldp igp sync' is for LDP-IGP synchronization, not SR-LDP interworking. Option D is incorrect because OSPF is not the IGP in use.
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.
✓
Configure 'segment-routing mpls sr-prefer' under the IS-IS process.
Why this is correct
This ensures the router prefers SR labels but still allocates LDP labels for backward compatibility.
Change the IGP to OSPF with segment routing enabled.
Why it's wrong here
OSPF is not the IGP in use; changing IGP would cause unnecessary complexity.
✗
Remove the 'mpls ldp' configuration from all routers that have SR enabled.
Why it's wrong here
Removing LDP globally would break connectivity for routers still relying on LDP.
✗
Enable 'mpls ldp igp sync' on the IGP interfaces.
Why it's wrong here
LDP-IGP sync prevents LDP from advertising a label until the IGP is fully converged, not SR-LDP interworking.
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: Configure 'segment-routing mpls sr-prefer' under the IS-IS process. — Option C is correct because during migration, both LDP and SR need to signal the same labels. The 'segment-routing mpls sr-prefer' command tells the router to prefer SR labels but still maintain LDP forwarding entries for fallback. Option A is incorrect because removing LDP globally would cause loss of LDP sessions for routers not yet SR-capable. Option B is incorrect because 'mpls ldp igp sync' is for LDP-IGP synchronization, not SR-LDP interworking. Option D is incorrect because OSPF is not the IGP in use.
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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