350-401 Network Assurance Practice Question
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?
⚠ Common exam trap
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.
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
✓
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.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
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Rendezvous Point (RP)
Why it's wrong here
An RP is a router role within PIM-SM that acts as the meeting point for sources and receivers to construct shared trees. Although a receiver's first-hop router sends PIM join messages toward the RP, that path originates from IGMP-learned host membership. Hosts are unaware of the RP's IP address and never interact with it directly; they simply send IGMP reports to their local gateway. Hence, RP is a routing-tree concept, not a host-to-router group discovery protocol.
- ✓
Internet Group Management Protocol (IGMP)
Why this is correct
IGMP is the end-system to router protocol that lets hosts on a directly connected subnet announce their interest in a specific multicast group, which is exactly what an IPTV receiver must do to request a channel. Routers send general queries and process membership reports to maintain an active group list on each interface. IGMPv3 further supports source-specific joins (S,G), enabling explicit control over which IPTV streams are received. Without IGMP, the first-hop router would have no way to know that a host wants multicast traffic.
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Protocol Independent Multicast (PIM)
Why it's wrong here
PIM is a family of multicast routing protocols (sparse-mode, dense-mode, bidirectional) that run between routers to build distribution trees and forward multicast traffic. It does not carry host membership information; routers learn local receivers through IGMP and then use PIM to implement the tree. Hosts do not exchange PIM messages with their access router. For IPTV, the host's group join is an IGMP report, not a PIM message.
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Multicast Source Discovery Protocol (MSDP)
Why it's wrong here
MSDP is an inter-domain protocol that allows Rendezvous Points in one PIM domain to advertise multicast sources to RPs in other PIM domains, enabling source discovery across complex topologies. It operates entirely between routers and has no role in host-to-router signaling. Hosts never run MSDP; they rely on IGMP to report their membership to the first-hop router. Therefore, MSDP cannot be the protocol used by an IPTV host to join a group.
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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.