The answer is that the prerequisite for MPLS L3VPN missing on PE1 is the VRF definition. This is correct because while the interface has an IP address and MPLS enabled, a VRF must be explicitly configured on the PE router to create a separate routing table for the customer. Without the VRF, the PE cannot isolate customer routes, import or export VPNv4 prefixes via MP-BGP, or maintain the per-VPN forwarding instance required for L3VPN operation. On the Cisco SPCOR 350-501 exam, this scenario tests your understanding that VRF configuration is a non-negotiable first step before any customer-facing interface can participate in an L3VPN; a common trap is assuming MPLS enablement alone is sufficient. Remember the memory tip: "No VRF, no VPN" — the VRF is the container that makes Layer 3 VPN separation possible.
350-501 Services Practice Question
This 350-501 practice question tests your understanding of services. 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.
Exhibit
Refer to the exhibit.
! On PE1
interface Loopback0
ip address 10.0.0.1 255.255.255.255
!
interface GigabitEthernet0/0/0
mpls ip
!
router ospf 1
router-id 10.0.0.1
network 10.0.0.0 0.255.255.255 area 0
network 192.168.0.0 0.0.255.255 area 0
!
router ldp
!
router bgp 65000
neighbor 10.0.0.2 remote-as 65000
neighbor 10.0.0.2 update-source Loopback0
!
address-family vpnv4
neighbor 10.0.0.2 activate
neighbor 10.0.0.2 send-community extended
!
Refer to the exhibit. Which prerequisite for MPLS L3VPN is missing on PE1?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
VRF is not defined
Option C is correct because the exhibit shows that PE1 has an interface configured with an IP address and MPLS enabled, but no VRF definition is present. For MPLS L3VPN, a VRF must be defined on the PE router to separate customer routing tables and to associate the customer-facing interface with that VRF. Without the VRF, the PE cannot import/export VPN routes or maintain per-VPN routing instances, which is a fundamental prerequisite for L3VPN operation.
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.
✗
MPLS is not enabled on the interface
Why it's wrong here
`mpls ip` is present.
✗
LDP is not enabled globally
Why it's wrong here
LDP is enabled with `router ldp`.
✓
VRF is not defined
Why this is correct
No `ip vrf` definition or VRF applied to any interface.
Related concept
Read the scenario before looking for a memorised answer.
✗
OSPF is not redistributed into BGP
Why it's wrong here
Redistribution is done within the VRF, but VRF itself is missing.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that MPLS or LDP configuration alone is sufficient for L3VPN, when in fact the VRF definition is the mandatory first step that candidates overlook.
Detailed technical explanation
How to think about this question
In MPLS L3VPN, the VRF (VPN Routing and Forwarding) instance creates a separate routing table on the PE, isolated from the global table. The VRF is associated with an interface using the 'ip vrf forwarding <vrf-name>' command, which clears the interface IP and requires reconfiguration. Without this, the PE cannot distinguish between customer routes or apply route distinguishers (RD) and route targets (RT) for VPNv4 route exchange via MP-BGP.
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.
Services — This question tests Services — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: VRF is not defined — Option C is correct because the exhibit shows that PE1 has an interface configured with an IP address and MPLS enabled, but no VRF definition is present. For MPLS L3VPN, a VRF must be defined on the PE router to separate customer routing tables and to associate the customer-facing interface with that VRF. Without the VRF, the PE cannot import/export VPN routes or maintain per-VPN routing instances, which is a fundamental prerequisite for L3VPN operation.
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.
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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