The answer is that all four bridges are mapped to the same physical interface without subinterfaces, creating a Layer 2 conflict. In Cisco NFVIS, each bridge acts as an isolated forwarding domain, but when multiple bridges share a single physical interface without unique subinterface tags, they collapse into the same VLAN and MAC domain, causing traffic from the second, third, and fourth bridges to be dropped or misrouted. This scenario tests your understanding of NFVIS bridge-to-interface mapping, a key concept for the ENCOR 350-401 exam’s virtualization section, where the common trap is assuming bridges automatically isolate traffic without proper subinterface segmentation. Remember that NFVIS treats a physical interface as a single broadcast domain; to support multiple bridges, you must create subinterfaces (e.g., GigabitEthernet0/0.10) and map each bridge to its own subinterface. A useful memory tip: one bridge, one interface or subinterface—no sharing without tagging.
350-401 Network Function Virtualization Practice Question
This 350-401 practice question tests your understanding of network function virtualization. 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.
An engineer is deploying a virtual network function (VNF) on a Cisco NFVIS host. The VNF requires four virtual NICs, each connected to a different network segment. The engineer creates four bridges on NFVIS and attaches each vNIC to a separate bridge. After deployment, the VNF can only communicate on the first bridge. 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: "first"
Why it matters: Order matters here. You are being tested on which action comes before the others — not which action is generally useful.
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 bridges are all mapped to the same physical interface without subinterfaces, causing a conflict.
In Cisco NFVIS, bridges are Layer 2 forwarding constructs that must be mapped to a physical interface (or subinterface) to provide external connectivity. When multiple bridges are all mapped to the same physical interface without using subinterfaces (e.g., GigabitEthernet0/0), they share the same VLAN and MAC domain, causing traffic from the second, third, and fourth bridges to be dropped or misdirected. The VNF can only communicate on the first bridge because that bridge's vNIC is the only one that successfully establishes a valid forwarding path through the physical interface.
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.
✓
The bridges are all mapped to the same physical interface without subinterfaces, causing a conflict.
Why this is correct
Correct because each bridge must be associated with a unique physical interface or subinterface; otherwise, only one bridge works.
Clue confirmation
The clue words "first", "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
The VNF's operating system does not support multiple NICs.
Why it's wrong here
Incorrect because most VNFs support multiple NICs; the issue is at the NFVIS bridge level.
✗
The vNICs have duplicate MAC addresses.
Why it's wrong here
Incorrect because NFVIS automatically assigns unique MAC addresses; duplicate MACs would cause issues on all interfaces.
✗
The bridges were created in the wrong order.
Why it's wrong here
Incorrect because the order of bridge creation does not affect their functionality.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that bridges in NFVIS are isolated by default, when in fact they require explicit mapping to unique physical interfaces or subinterfaces to avoid Layer 2 conflicts.
Detailed technical explanation
How to think about this question
Under the hood, each NFVIS bridge acts as a virtual switch that must be bound to a specific physical interface or subinterface using the 'bridge map' command. Without subinterfaces (e.g., GigabitEthernet0/0.100, GigabitEthernet0/0.200), all bridges map to the same untagged VLAN on the physical port, causing a single collision domain where only the first bridge's forwarding table is correctly populated. In real-world deployments, this is avoided by using 802.1Q subinterfaces or dedicated physical interfaces per bridge to isolate traffic segments.
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.
Network Function Virtualization — This question tests Network Function Virtualization — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The bridges are all mapped to the same physical interface without subinterfaces, causing a conflict. — In Cisco NFVIS, bridges are Layer 2 forwarding constructs that must be mapped to a physical interface (or subinterface) to provide external connectivity. When multiple bridges are all mapped to the same physical interface without using subinterfaces (e.g., GigabitEthernet0/0), they share the same VLAN and MAC domain, causing traffic from the second, third, and fourth bridges to be dropped or misdirected. The VNF can only communicate on the first bridge because that bridge's vNIC is the only one that successfully establishes a valid forwarding path through the physical interface.
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.
Are there clue words in this question I should notice?
Yes — watch for: "first", "most likely". Order matters here. You are being tested on which action comes before the others — not which action is generally useful.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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