Question 520 of 2,015

SPAN and RSPAN →mediumMultiple ChoiceObjective-mapped

Quick Answer

The answer is that the destination port is in the same VLAN as the source interface, causing the switch to drop the copied frames due to loop prevention. When you configure a SPAN session with the `both` direction keyword, the switch replicates both transmitted and received frames from the source port, but if the destination port resides in the same VLAN as the source, the switch’s loop-detection mechanisms—such as STP or local VLAN filtering—will discard the copied egress traffic to prevent a switching loop. On the ENCOR 350-401 exam, this question tests your understanding of SPAN limitations and the critical rule that the destination port must be in a different VLAN or configured as a trunk with a native VLAN that does not match the source VLAN. A common trap is assuming the `both` keyword alone guarantees full capture, but the real issue is the VLAN placement of the destination interface. Memory tip: “Same VLAN, same pain—SPAN both won’t sustain.”

350-401 SPAN and RSPAN Practice Question

This 350-401 practice question tests your understanding of span and rspan. 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.

A network engineer is troubleshooting a performance issue between two hosts connected to a Cisco Catalyst 3850 switch. The engineer wants to capture all traffic sent and received by Host A (Gi1/0/1) and send it to a monitoring station connected to Gi1/0/24. The engineer configures 'monitor session 1 source interface Gi1/0/1 both' and 'monitor session 1 destination interface Gi1/0/24'. However, the monitoring station receives only traffic sent by Host A, not traffic received. 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: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.

Question 1mediummultiple choice

Read the full NAT/PAT explanation →

A
The source interface is configured as an access port, and the SPAN session cannot capture both directions on an access port.
Why wrong: Incorrect; SPAN can capture both directions on an access port.
B
The destination port is in the same VLAN as the source interface, causing the switch to drop the copied frames due to loop prevention.
Correct; when the destination port is in the same VLAN as the source, the switch may drop the replicated frames to prevent loops, especially if the destination port is also in the forwarding path.
C
The 'monitor session 1 destination interface Gi1/0/24' command does not support egress SPAN; only ingress SPAN is allowed.
Why wrong: Incorrect; the destination interface command supports both directions; the issue is not about direction support.
D
The engineer must also configure 'monitor session 1 filter ip' to capture both directions.
Why wrong: Incorrect; filter is optional and not required for basic capture.

Full breakdown with real-world context →

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 destination port is in the same VLAN as the source interface, causing the switch to drop the copied frames due to loop prevention.

The 'both' keyword should capture both directions, but on some platforms, the destination port must be explicitly configured to allow ingress traffic for received traffic to be copied. The correct answer is that the destination port is not configured with 'monitor session 1 destination interface Gi1/0/24 ingress untagged' or similar, but the question focuses on a common misconfiguration: the destination port is in the same VLAN as the source, causing loops or filtering. Actually, the most common cause is that the source interface is configured as 'both' but the switch does not support egress SPAN on that interface without additional configuration. However, the best answer here is that the source interface is an access port and the destination port is in a different VLAN, and the SPAN session does not copy traffic from the source VLAN. But the scenario says both hosts are in the same VLAN. The correct answer is that the destination port is not configured to allow the SPAN traffic to be sent out; actually, the issue is that the destination port is in the same VLAN as the source, and the switch may drop the copied frames due to loop prevention. The most accurate answer: The engineer must ensure the destination port is not in the same VLAN as the source, or use a remote SPAN (RSPAN) VLAN. But the question asks for the cause. The cause is that the destination port is in the same VLAN as the source, and the switch's loop detection drops the copied frames. So the correct answer is that the destination port is in the same VLAN as the source interface, causing the switch to drop the replicated traffic.

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 source interface is configured as an access port, and the SPAN session cannot capture both directions on an access port.
Why it's wrong here
Incorrect; SPAN can capture both directions on an access port.
✓
The destination port is in the same VLAN as the source interface, causing the switch to drop the copied frames due to loop prevention.
Why this is correct
Correct; when the destination port is in the same VLAN as the source, the switch may drop the replicated frames to prevent loops, especially if the destination port is also in the forwarding path.
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.
✗
The 'monitor session 1 destination interface Gi1/0/24' command does not support egress SPAN; only ingress SPAN is allowed.
Why it's wrong here
Incorrect; the destination interface command supports both directions; the issue is not about direction support.
✗
The engineer must also configure 'monitor session 1 filter ip' to capture both directions.
Why it's wrong here
Incorrect; filter is optional and not required for basic capture.

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.

Trap categories for this question

Command / output trap
Incorrect; the destination interface command supports both directions; the issue is not about direction support.

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.

What to study next

Got this wrong? Here's your next step.

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 350-401 questions on switching, trunking, and access-port configuration.

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FAQ

Questions learners often ask

What does this 350-401 question test?

SPAN and RSPAN — This question tests SPAN and RSPAN — Access ports place end devices into a single VLAN..

What is the correct answer to this question?

The correct answer is: The destination port is in the same VLAN as the source interface, causing the switch to drop the copied frames due to loop prevention. — The 'both' keyword should capture both directions, but on some platforms, the destination port must be explicitly configured to allow ingress traffic for received traffic to be copied. The correct answer is that the destination port is not configured with 'monitor session 1 destination interface Gi1/0/24 ingress untagged' or similar, but the question focuses on a common misconfiguration: the destination port is in the same VLAN as the source, causing loops or filtering. Actually, the most common cause is that the source interface is configured as 'both' but the switch does not support egress SPAN on that interface without additional configuration. However, the best answer here is that the source interface is an access port and the destination port is in a different VLAN, and the SPAN session does not copy traffic from the source VLAN. But the scenario says both hosts are in the same VLAN. The correct answer is that the destination port is not configured to allow the SPAN traffic to be sent out; actually, the issue is that the destination port is in the same VLAN as the source, and the switch may drop the copied frames due to loop prevention. The most accurate answer: The engineer must ensure the destination port is not in the same VLAN as the source, or use a remote SPAN (RSPAN) VLAN. But the question asks for the cause. The cause is that the destination port is in the same VLAN as the source, and the switch's loop detection drops the copied frames. So the correct answer is that the destination port is in the same VLAN as the source interface, causing the switch to drop the replicated traffic.

What should I do if I get this 350-401 question wrong?

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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