The answer is that the IP prefix is not resolved via an MPLS label in the LFIB, which is the most likely cause of the punt path. When a VRF route shows "punt" in the show cef vrf output on an ASR 9000, it means the Forwarding Information Base (FIB) cannot find a valid MPLS label for the next hop, forcing the packet to be processed by the CPU rather than being label-switched. This scenario directly tests your understanding of MPLS L3VPN punt path troubleshooting, a key topic on the Cisco SPCOR 350-501 exam, where candidates must differentiate between control-plane and data-plane forwarding failures. A common trap is assuming BGP VPNv4 routes are needed for local VRF forwarding, but the real issue is a missing label binding in the Label Forwarding Information Base (LFIB). Remember the memory tip: "Punt means no label in the LFIB—check your LDP or targeted LDP sessions first."
350-501 Architecture Practice Question
This 350-501 practice question tests your understanding of architecture. 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.
A service provider is troubleshooting an MPLS L3VPN where customers behind CE-A cannot reach CE-B. The PE routers are Cisco ASR 9000 series. On PE-A, the show cef vrf CUSTOMER prefix 10.1.1.0/24 command displays 'punt' as the forwarding path. 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 IP prefix is not resolved via an MPLS label in the LFIB
Option C is correct because when VRF routes are punted to the CPU, it typically indicates that the label for the next hop is missing or that the FIB resolution failed, often due to missing MPLS label binding. Option A is wrong because BGP VPNv4 routes are not required for local VRF forwarding. Option B is wrong because MTU would cause drop, not punt. Option D is wrong because CE-PE routing is usually static or BGP, not OSPF in most designs.
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 IP prefix is not resolved via an MPLS label in the LFIB
Why this is correct
Punt indicates the CEF cannot find a label for the next hop; likely missing LDP adjacency.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
The VRF is missing the route-target import statement
Why it's wrong here
Route-target import affects BGP VPNv4 route reception, not local FIB resolution.
✗
The CE is not running OSPF with the PE
Why it's wrong here
CE-PE routing could be any protocol; OSPF not required.
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.
Architecture — This question tests Architecture — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: The IP prefix is not resolved via an MPLS label in the LFIB — Option C is correct because when VRF routes are punted to the CPU, it typically indicates that the label for the next hop is missing or that the FIB resolution failed, often due to missing MPLS label binding. Option A is wrong because BGP VPNv4 routes are not required for local VRF forwarding. Option B is wrong because MTU would cause drop, not punt. Option D is wrong because CE-PE routing is usually static or BGP, not OSPF in most designs.
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.
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?
OSPF neighbours must agree on key parameters.
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