mediumMultiple Select
350-401 Practice Question: Which two statements about IGMP snooping are…
Which two statements about IGMP snooping are true? (Choose two.)
⚠ Common exam trap
Many exam-takers confuse IGMP snooping (Layer 2 optimization that listens to IGMP) with IGMP itself (Layer 3 group management) — candidates often pick the querier-election option because they conflate the two protocols.
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
✓
IGMP snooping reduces unnecessary multicast flooding on a Layer 2 switch.
Option A is correct because IGMP snooping allows a Layer 2 switch to examine IGMP messages and forward multicast traffic only to ports that have interested receivers, rather than flooding it to all ports in the VLAN. Option D is correct because the switch snoops IGMP membership reports and leave messages to dynamically build and maintain a Layer 2 multicast forwarding table mapping groups to ports. Option B is incorrect because querier election is an IGMP function performed by multicast routers (or a snooping switch acting as querier), not a mechanism that defines IGMP snooping itself. Option C is incorrect because IGMP snooping operates transparently on the switch and does not replace IGMP on router interfaces. Option E is incorrect because IGMP snooping does not modify IP headers; it only inspects IGMP control messages to build forwarding state.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
IGMP snooping reduces unnecessary multicast flooding on a Layer 2 switch.
Why this is correct
IGMP snooping lets a Layer 2 switch inspect IGMP join and leave messages, building a per-VLAN multicast forwarding table. Frames are then forwarded only to ports with interested receivers, satisfying the stem's requirement to reduce unnecessary multicast flooding.
- ✗
IGMP snooping uses the IGMP querier election process to select the switch with the highest IP address as the querier.
Why it's wrong here
Querier election is an IGMP function performed by multicast routers; the switch only snoops the resulting queries, so it never elects a querier by highest IP address. It tempts because IGMP snooping does involve querier messages, and on a router interface the highest-IP election genuinely applies.
- ✗
IGMP snooping can be configured to replace IGMP on the router interface.
Why it's wrong here
Snooping is a Layer 2 optimisation that constrains multicast flooding; the router interface still runs IGMP to manage group membership, so snooping cannot replace it. It tempts because snooping reduces unnecessary multicast traffic, and on a switch without any multicast router, querier configuration is genuinely needed.
- ✓
IGMP snooping listens to IGMP membership reports and leave messages to build a forwarding table.
Why this is correct
IGMP snooping inspects IGMP membership reports and leave messages traversing the switch, using them to populate a Layer 2 multicast forwarding table mapping group addresses to interested ports, so traffic is forwarded only where requested.
- ✗
IGMP snooping modifies the IP header of multicast packets to include group membership information.
Why it's wrong here
IGMP snooping operates at Layer 2, inspecting IGMP messages and building forwarding tables; it never rewrites the IP header of multicast data packets. It tempts because snooping does track group membership, and a router running IGMP proxy would be the device that actually modifies membership signalling.
Visual reference
Go deeper
Related to this question
About these practice questions
Courseiva writes every 350-401 question from scratch — 1,923 in total, each with an explanation and a wrong-answer breakdown. None are copied from real exams or dumps. Learn why practice questions differ from exam dumps →
Same concept, more angles
1 more way this is tested on 350-401
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. Which three statements about IGMP snooping are true? (Choose three.)
medium- ✓ A.IGMP snooping reduces multicast flooding on a VLAN by forwarding traffic only to ports that have joined the group.
- ✓ B.IGMP snooping builds a Layer 2 forwarding table that maps multicast MAC addresses to switch ports.
- ✓ C.IGMP snooping forwards IGMP membership reports only toward the multicast router (querier).
- D.IGMP snooping suppresses all IGMP general queries from the multicast router.
- E.IGMP snooping requires PIM to be enabled on the switch.
Why A: Option A is correct because IGMP snooping constrains multicast traffic on a VLAN to only those switch ports that have sent IGMP membership reports for the group, instead of flooding frames to every port. Option B is correct because the switch maintains a Layer 2 multicast forwarding table (often called the IGMP snooping table) that maps the multicast MAC address (derived from the group's Class D IP address) to the ports that joined the group. Option C is correct because the switch acts as an IGMP proxy-like relay, forwarding membership reports only toward the multicast router/querier port so the router learns group membership without receiving reports from every host. Option D is not correct because general queries from the querier must be flooded to all ports in the VLAN so hosts can respond and maintain their memberships; suppressing them would break snooping. Option E is not correct because IGMP snooping operates at Layer 2 and does not require PIM, which is a Layer 3 multicast routing protocol.
JA
Written and reviewed by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
Last reviewed September 2026 · checked against the official Cisco exam blueprint
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.