The answer is a missing MPLS label in the LFIB on PE2. When a prefix appears in the VRF routing table (RIB) but not in the VRF forwarding table (FIB), it means the route was learned via MP-BGP and installed in the RIB, but the CEF process cannot populate the FIB entry because the required inner VPN label is absent from the Label Forwarding Information Base (LFIB). Without this label, the PE cannot encapsulate the packet for transport across the MPLS core, so the route remains in the RIB but is not forwarded. On the Cisco SPCOR 350-501 exam, this scenario tests your understanding of the MPLS L3VPN control plane versus data plane—specifically how BGP installs routes but CEF requires a valid label to forward. A common trap is to blame OSPF or LDP, but those distribute core labels, not the VPN label; the issue is always the missing per-VRF label in the LFIB. Memory tip: “RIB has the map, FIB needs the stamp—no label, no forward.”
350-501 MPLS and Segment Routing Practice Question
This 350-501 practice question tests your understanding of mpls and segment routing. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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.
An engineer is troubleshooting an MPLS L3VPN where customers behind CE1 cannot reach a specific prefix behind CE2. The PE routers are using OSPF as the IGP and LDP for label distribution. On PE2, the prefix is present in the VRF routing table, but not in the VRF forwarding table. What is the most likely cause?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The route is missing a label in the LFIB on PE2.
The prefix is present in the VRF routing table (RIB) but missing from the VRF forwarding table (FIB) on PE2. This indicates that the route has been learned via BGP and installed in the RIB, but the MPLS VPN label (the inner label) required to forward the packet across the MPLS core is absent. Without a valid label in the LFIB, the CEF (FIB) cannot install the route, causing the reachability failure. Option D correctly identifies this missing label in the LFIB as the root cause.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
MTU mismatch is causing the VPN label to be dropped.
Why it's wrong here
MTU mismatch would cause packet drops, not prevent the route from being installed in the forwarding table.
✗
OSPF is not redistributing the BGP routes into the IGP on PE2.
Why it's wrong here
OSPF redistribution is not required for MPLS label allocation; labels are assigned via LDP.
✗
The VRF is not properly configured on PE2's interface toward CE2.
Why it's wrong here
If the VRF were misconfigured, the route would not appear in the VRF routing table either.
✓
The route is missing a label in the LFIB on PE2.
Why this is correct
If the label is missing, the route cannot be installed in the VRF forwarding table.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the distinction between the routing table (RIB) and the forwarding table (FIB) in MPLS VPNs, trapping candidates who assume that a route present in the RIB automatically means it is usable for forwarding, when in fact the FIB requires a valid label binding to install the route.
Detailed technical explanation
How to think about this question
In MPLS L3VPN, when a PE receives a VPNv4 prefix from a remote PE via MP-BGP, it must have a valid MPLS label (the VPN label) in the LFIB to install the route into the VRF FIB. This label is assigned by the remote PE and advertised in the MP-BGP update. If the local PE cannot resolve the BGP next hop (e.g., due to missing LDP label for the transport LSP) or if the label is not properly programmed in the LFIB (e.g., due to a control-plane issue like a missing LDP session or a label allocation failure), the route remains in the RIB but is not forwarded. A common real-world scenario is a missing LDP label for the BGP next-hop address, which prevents the imposition of the transport label stack, causing the VPN label to be unusable.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
MPLS and Segment Routing — This question tests MPLS and Segment Routing — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The route is missing a label in the LFIB on PE2. — The prefix is present in the VRF routing table (RIB) but missing from the VRF forwarding table (FIB) on PE2. This indicates that the route has been learned via BGP and installed in the RIB, but the MPLS VPN label (the inner label) required to forward the packet across the MPLS core is absent. Without a valid label in the LFIB, the CEF (FIB) cannot install the route, causing the reachability failure. Option D correctly identifies this missing label in the LFIB as the root cause.
What should I do if I get this 350-501 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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