The most likely cause of the multicast flooding is that the IGMP querier is not elected on the VLAN. Without an elected querier, the switch never receives the IGMP membership reports from hosts, because no device is periodically sending the general queries that trigger those reports. IGMP snooping relies entirely on the switch CPU inspecting these reports to build its forwarding table; if the reports never arrive, the switch has no record of which ports want the multicast stream and must flood the traffic to all ports in the VLAN as a safety mechanism. On the ENCOR 350-401 exam, this scenario tests your understanding that IGMP snooping is not a standalone fix—it requires a functional querier, which can be a router or the switch itself if configured with the ip igmp snooping querier command. A common trap is assuming that enabling IGMP snooping globally and per-VLAN is sufficient, but without a querier, snooping is blind. Memory tip: No querier, no reports, full flood.
CCNP IP Multicast Practice Question
This 350-401 practice question tests your understanding of ip multicast. 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 is configuring multicast on a Cisco switch running IOS. The switch is acting as the IGMP querier for a VLAN. The engineer notices that multicast traffic is being flooded to all ports in the VLAN, even though only a few receivers have joined the group. The engineer checks the IGMP snooping configuration and sees that IGMP snooping is enabled globally and on the VLAN. What is the most likely cause of the flooding?
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 IGMP querier is not elected on the VLAN.
IGMP snooping relies on the switch seeing IGMP membership reports. If the switch does not see the reports because the querier is not elected or because the reports are not forwarded to the switch CPU, the switch will flood multicast traffic to all ports.
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 IGMP querier is not elected on the VLAN.
Why this is correct
Correct because without a querier, IGMP snooping cannot learn group memberships, causing the switch to flood multicast traffic.
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 multicast source is connected to a trunk port.
Why it's wrong here
Incorrect because the source port type does not directly cause flooding; IGMP snooping works on trunk ports as well.
✗
The switch has PIM enabled on the VLAN interface.
Why it's wrong here
Incorrect because PIM is for Layer 3 multicast routing, not for Layer 2 IGMP snooping.
✗
The receivers are using IGMPv3.
Why it's wrong here
Incorrect because IGMPv3 is supported by IGMP snooping and does not cause flooding.
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.
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.
IP Multicast — This question tests IP Multicast — Access ports place end devices into a single VLAN..
What is the correct answer to this question?
The correct answer is: The IGMP querier is not elected on the VLAN. — IGMP snooping relies on the switch seeing IGMP membership reports. If the switch does not see the reports because the querier is not elected or because the reports are not forwarded to the switch CPU, the switch will flood multicast traffic to all ports.
What should I do if I get this 350-401 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 350-401 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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