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300-410 Practice Question: An engineer configures SPAN on a Cisco switch to…
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?
Answer choices
Why each option matters
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.
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.
- ✗
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.
Why it's wrong here
SPAN captures native VLAN traffic if the source is a trunk port or VLAN.
- ✗
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.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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