The answer is Virtual Routing and Forwarding (VRF), which is the technology used to maintain customer traffic separation in the MPLS core. VRF works by creating unique, isolated routing tables and forwarding instances on the Provider Edge (PE) router for each customer, so that traffic from one customer’s VPN never leaks into another’s routing domain—this separation is enforced at Layer 3, independent of the MPLS label switching that occurs in the core. On the ENCOR 350-401 exam, this concept tests your understanding of how service providers achieve multi-tenant isolation without relying on separate physical infrastructure; a common trap is confusing VRF with VLANs (Layer 2) or assuming MPLS labels alone provide the separation. Remember the memory tip: VRF = Virtual Routing and Forwarding, meaning each customer gets their own virtual router inside the PE, keeping routes and forwarding completely separate.
350-401 Architecture Practice Question
This 350-401 practice question tests your understanding of architecture. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. 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 company is deploying a WAN with MPLS VPN and wants to ensure that customer traffic is isolated from other customers. Which technology is used to maintain separation in the MPLS core?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Virtual Routing and Forwarding (VRF)
VRF (Virtual Routing and Forwarding) is the technology used in MPLS VPN to maintain customer traffic separation within the MPLS core. Each customer is assigned a unique VRF on the Provider Edge (PE) router, which maintains a separate routing table and forwarding instance, ensuring that traffic from one customer never crosses into another customer's routing domain. This separation is enforced at Layer 3, independent of the MPLS label switching that occurs in the core.
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.
✗
VLAN tagging
Why it's wrong here
VLAN tagging provides Layer 2 isolation but not at Layer 3 in MPLS core.
✗
MPLS labels
Why it's wrong here
MPLS labels are used for forwarding but do not provide routing isolation.
✗
IPsec tunnels
Why it's wrong here
IPsec provides encryption and integrity, but not routing isolation between customers.
✓
Virtual Routing and Forwarding (VRF)
Why this is correct
VRF creates separate routing instances per VPN customer, maintaining traffic isolation.
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 misconception that MPLS labels alone provide customer separation, but labels are only a forwarding mechanism; the actual isolation comes from VRF instances on the PE routers.
Detailed technical explanation
How to think about this question
In MPLS Layer 3 VPN (RFC 4364), each VRF is associated with a unique Route Distinguisher (RD) and Route Target (RT) to ensure that routes from different customers remain isolated even if they use overlapping IP addresses. The PE router maintains a separate VRF for each customer, and when a packet enters the MPLS core, it is encapsulated with two labels: the inner label (VPN label) identifies the egress VRF, and the outer label (transport label) is used for core switching. A common real-world scenario is when two customers use the same RFC 1918 address space (e.g., 10.0.0.0/8); VRF allows them to coexist without conflict.
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 help-desk technician troubleshoots why a newly connected PC cannot reach shared printers on the same floor. The cable is good, the switch port is active, but the PC is in VLAN 20 and the printers are in VLAN 10. The uplink trunk only allows VLAN 10. A trunk being up does not mean every VLAN crosses it.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 350-401 question in full detail.
Architecture — This question tests Architecture — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Virtual Routing and Forwarding (VRF) — VRF (Virtual Routing and Forwarding) is the technology used in MPLS VPN to maintain customer traffic separation within the MPLS core. Each customer is assigned a unique VRF on the Provider Edge (PE) router, which maintains a separate routing table and forwarding instance, ensuring that traffic from one customer never crosses into another customer's routing domain. This separation is enforced at Layer 3, independent of the MPLS label switching that occurs in the core.
What should I do if I get this 350-401 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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