The correct protocol is the Internet Group Management Protocol (IGMP), which routers use to discover multicast groups on directly connected hosts. IGMP operates between hosts and their first-hop router, allowing hosts to signal their interest in specific multicast streams by sending membership reports; the router then uses these reports to build and maintain its multicast forwarding state for that subnet. On the ENCOR 350-401 exam, this concept tests your understanding of Layer 3 multicast signaling and the distinction between host-to-router protocols (IGMP) and router-to-router protocols (PIM). A common trap is confusing IGMP with PIM—remember that IGMP handles group discovery on the local link, while PIM handles multicast routing between networks. Memory tip: IGMP = "I Get Membership from People" (hosts), while PIM = "Pass It to My neighbors" (routers).
350-401 Network Assurance Practice Question
This 350-401 practice question tests your understanding of network assurance. Match the stated requirement to the specific cloud service, access model, or configuration option — many options are valid in isolation but not for this scenario. 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 designing a multicast network for IPTV. Which protocol is used by routers to discover which multicast groups are of interest to directly connected hosts?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Internet Group Management Protocol (IGMP)
IGMP is the protocol used between hosts and their directly connected routers to signal membership in multicast groups. When a host wants to receive traffic for a specific IPTV multicast stream, it sends an IGMP membership report, and the router uses this information to build its multicast forwarding state for that subnet. Without IGMP, the router would have no way of knowing which groups are of interest to local hosts.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
Rendezvous Point (RP)
Why it's wrong here
Option C is wrong because RP is a function, not a protocol.
✓
Internet Group Management Protocol (IGMP)
Why this is correct
Option D is correct because IGMP is used by hosts to report group membership to routers.
Related concept
Read the scenario before looking for a memorised answer.
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Protocol Independent Multicast (PIM)
Why it's wrong here
Option A is wrong because PIM is used for routing multicast traffic between routers.
✗
Multicast Source Discovery Protocol (MSDP)
Why it's wrong here
Option B is wrong because MSDP is used to exchange source information between RPs.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the distinction between host-to-router signaling (IGMP) and router-to-router multicast routing (PIM), so candidates mistakenly choose PIM when the question explicitly asks about discovering groups of interest to directly connected hosts.
Detailed technical explanation
How to think about this question
IGMP operates at the network layer (IP protocol number 2) and has three versions: IGMPv2 (RFC 2236) adds leave-group messaging, while IGMPv3 (RFC 3376) adds source-specific multicast (SSM) support, allowing hosts to specify both group and source addresses. The router uses IGMP snooping at Layer 2 to constrain multicast traffic to only those switch ports that have interested receivers, which is critical in IPTV deployments to avoid flooding all ports.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A network engineer segments a warehouse floor into three subnets: 20 scanners, 5 printers, and 2 management hosts. Picking the wrong mask wastes addresses or leaves too few usable hosts. Exam questions test whether you can apply CIDR notation, calculate block size, and identify the correct usable-host range for a given prefix.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 350-401 question in full detail.
Network Assurance — This question tests Network Assurance — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Internet Group Management Protocol (IGMP) — IGMP is the protocol used between hosts and their directly connected routers to signal membership in multicast groups. When a host wants to receive traffic for a specific IPTV multicast stream, it sends an IGMP membership report, and the router uses this information to build its multicast forwarding state for that subnet. Without IGMP, the router would have no way of knowing which groups are of interest to local hosts.
What should I do if I get this 350-401 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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