The answer is to configure `label mode per-prefix` under the BGP neighbor or address-family. This is required because, by default, BGP-LU allocates labels on a per-next-hop basis, which means the label is shared across all prefixes using that next hop, preventing individual prefix installation into the LFIB. Setting `label mode per-prefix` forces BGP to allocate a unique label for each prefix, allowing the MPLS forwarding table to program the label-switched path for those BGP-LU routes. On the Cisco SPCOR 350-501 exam, this scenario tests your understanding of the interaction between BGP label allocation and MPLS forwarding—a common trap is assuming that a label in the BGP table guarantees LFIB installation, but without per-prefix mode or `mpls bgp forwarding`, the LFIB remains empty. Memory tip: "Per prefix, per path—without it, LFIB is a ghost."
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 network engineer notices that BGP-LU prefixes learned from an eBGP neighbor are not being installed in the MPLS forwarding table (LFIB). The BGP table shows the prefixes with the correct label. The IGP route to the neighbor is reachable. What additional configuration is needed?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Set `label mode per-prefix` under the BGP neighbor or address-family
For BGP-LU to install labels into LFIB, the `label mode` must be set to `per-prefix` or the router must have `mpls bgp forwarding` enabled. Without enabling label allocation per prefix, BGP-LU prefixes are not programmed in LFIB. The other options are not directly related.
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.
✗
Enable `no bgp default ipv4-unicast`
Why it's wrong here
That command prevents IPv4 unicast from being advertised, not label installation.
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.
Trap categories for this question
Command / output trap
That command prevents IPv4 unicast from being advertised, not label installation.
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: Set `label mode per-prefix` under the BGP neighbor or address-family — For BGP-LU to install labels into LFIB, the `label mode` must be set to `per-prefix` or the router must have `mpls bgp forwarding` enabled. Without enabling label allocation per prefix, BGP-LU prefixes are not programmed in LFIB. The other options are not directly related.
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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