The answer is that the SPAN source is configured for the data VLAN only, not the voice VLAN. This occurs because voice VLANs are typically separate from data VLANs on a Cisco switch—for example, VLAN 100 for voice and VLAN 10 for data—so a SPAN session targeting a single VLAN will only capture traffic on that specific VLAN, omitting the voice traffic entirely. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding of SPAN source selection and the common misconfiguration where engineers forget that voice and data coexist on the same access port but belong to different VLANs. A frequent trap is assuming that mirroring the access VLAN will also capture the voice VLAN, but the switch treats them as distinct Layer 2 domains. To capture both, you must either include both VLANs in the SPAN session or use a port-based source that spans all VLANs on the interface. Memory tip: “Voice and data are VLAN neighbors, not roommates—SPAN needs an invitation to both.”
300-410 SPAN, RSPAN, and ERSPAN Practice Question
This 300-410 practice question tests your understanding of span, rspan, and erspan. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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 configures SPAN on a Cisco switch to monitor traffic from a VLAN, but the VLAN includes a voice VLAN. The mirrored traffic shows only data traffic, not voice. What is the most likely explanation?
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 SPAN source is configured for the data VLAN only, not the voice VLAN.
Voice VLANs are often configured as separate VLANs (e.g., VLAN 100 for voice, VLAN 10 for data). If the SPAN source is a single VLAN, it only captures traffic on that VLAN. To capture both, the SPAN session must include both VLANs or use a port-based source that includes both.
Key principle: A trunk being up does not mean the VLAN is allowed across it. Always verify the allowed VLAN list and whether the VLAN exists on both switches.
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 SPAN source is configured for the data VLAN only, not the voice VLAN.
Why this is correct
SPAN VLAN-based monitoring captures only the specified VLAN; the voice VLAN must be added separately.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Access ports place end devices into a single VLAN.
✗
Voice traffic uses a different CoS value, which SPAN ignores.
Why it's wrong here
SPAN does not filter by CoS; it captures all traffic in the VLAN.
✗
The voice VLAN is configured as a native VLAN, which SPAN does not capture.
The switch has a security ACL that blocks voice traffic from being mirrored.
Why it's wrong here
ACLs do not affect SPAN; they affect forwarding.
Common exam traps
Common exam trap: an active trunk can still block the VLAN you need
A trunk being up does not prove every VLAN is crossing it. Check allowed VLAN lists, native VLAN mismatch, VLAN existence and access-port assignment.
Detailed technical explanation
How to think about this question
VLAN questions usually combine access-port and trunking clues. The key is to identify whether the issue is local to one switchport, caused by the trunk, or caused by the VLAN not existing where it needs to exist.
KKey Concepts to Remember
Access ports place end devices into a single VLAN.
Trunk ports carry multiple VLANs between switches.
Allowed VLAN lists decide which VLANs can cross a trunk.
Native VLAN mismatch can create confusing symptoms.
TExam Day Tips
→Use show vlan brief to verify access VLANs.
→Use show interfaces trunk to verify trunk state and allowed VLANs.
→Do not treat every same-VLAN issue as a routing problem.
Key takeaway
A trunk being up does not mean the VLAN is allowed across it. Always verify the allowed VLAN list and whether the VLAN exists on both switches.
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 300-410 question in full detail.
Review VLAN allowed lists, native VLAN mismatch detection, and how to verify VLAN membership with show vlan brief and show interfaces trunk. Then practise related 300-410 questions on switching, trunking, and access-port configuration.
SPAN, RSPAN, and ERSPAN — This question tests SPAN, RSPAN, and ERSPAN — Access ports place end devices into a single VLAN..
What is the correct answer to this question?
The correct answer is: The SPAN source is configured for the data VLAN only, not the voice VLAN. — Voice VLANs are often configured as separate VLANs (e.g., VLAN 100 for voice, VLAN 10 for data). If the SPAN source is a single VLAN, it only captures traffic on that VLAN. To capture both, the SPAN session must include both VLANs or use a port-based source that includes both.
What should I do if I get this 300-410 question wrong?
Review VLAN allowed lists, native VLAN mismatch detection, and how to verify VLAN membership with show vlan brief and show interfaces trunk. Then practise related 300-410 questions on switching, trunking, and access-port configuration.
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?
Access ports place end devices into a single VLAN.
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