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350-401 Practice Question: An engineer is configuring multicast on a Cisco…
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?
⚠ Common exam trap
Cisco often tests the misconception that enabling IGMP snooping alone is sufficient to prevent flooding, without realizing that an IGMP querier must be present on the VLAN to populate the snooping table.
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 IGMP querier is not elected on the VLAN.
When IGMP snooping is enabled but no IGMP querier exists on the VLAN, the switch cannot learn which ports have interested receivers because no general queries are sent. Without querier-generated queries, the snooping database remains empty, causing the switch to flood multicast traffic to all ports in the VLAN as if snooping were disabled.
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
IGMP snooping builds and refreshes its forwarding table only when IGMP queries are present on the VLAN to solicit membership reports from receivers. If no IGMP querier is elected on the VLAN (for example, no multicast router is enabled to send general queries and the switch's querier feature is disabled), the switch never receives periodic reports and cannot maintain the multicast group, so it floods multicast frames to every port. This is the direct cause of the flooding problem.
- ✗
The multicast source is connected to a trunk port.
Why it's wrong here
The type of port that connects the multicast source is irrelevant to IGMP snooping, because snooping inspects the IGMP messages in frame payloads and builds an MDB keyed by group and receiving VLAN, not by ingress port mode. A trunk port can carry multicast traffic perfectly well, and IGMP snooping still creates the necessary forwarding state for that traffic. The failure to elect an IGMP querier is the reason the switch floods, not the presence of a trunk port.
- ✗
The switch has PIM enabled on the VLAN interface.
Why it's wrong here
PIM runs at Layer 3 to build multicast routing tables between routers, using hello messages and RPF checks; enabling it on the VLAN SVI does not affect Layer 2 IGMP snooping operations. While PIM and IGMP are often configured together, PIM does not provide the IGMP querying function that the switch needs to learn group memberships from receivers. Therefore, PIM being enabled on the VLAN interface cannot explain the flooding behavior.
- ✗
The receivers are using IGMPv3.
Why it's wrong here
IGMPv3 is fully supported by IGMP snooping, and the switch can parse IGMPv3 membership reports and group address records to populate the snooping table. Receivers using IGMPv3 simply send reports with source filters, but this does not change the requirement for an IGMP querier to periodically send general queries that trigger those reports. Flooding is caused by the absence of a querier, not by the IGMP version used by receivers.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 350-401 practice question is part of Courseiva's free Cisco certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 350-401 exam.