Questions: IGMP (Internet Group Management Protocol)

Question 1

A host on a LAN segment sends an IGMP Membership Report for group 239.1.1.1. Which device's behavior is directly affected by this message?

• A. The multicast source, which will now start sending traffic to the host
• B. The first-hop router on the same LAN segment, which will start forwarding group traffic to that link
• C. All routers on the Internet that handle multicast routing for group 239.1.1.1
• D. The DNS server, which will resolve the group address to a hostname

Answer: (B)

IGMP operates only between hosts and their local (first-hop) router on the same link. The Membership Report tells that router 'at least one host on my link wants group 239.1.1.1,' causing the router to start forwarding that group's traffic to the link. IGMP does NOT communicate with the multicast source, with distant routers, or with DNS. Inter-router multicast distribution is handled separately by multicast routing protocols like PIM, which build trees based on the local IGMP membership information.

Question 2

In IGMPv2, a host is receiving traffic for multicast group 224.2.2.2. It decides to leave the group. What sequence of events follows?

• A. The host simply stops listening; the router detects this when its next membership report times out
• B. The host sends a Leave Group message; the router may send a Group-Specific Query; if no other hosts respond, forwarding stops
• C. The host sends an IGMP Membership Report for a different group to override its previous report
• D. The host must contact the multicast source directly to unsubscribe

Answer: (B)

IGMPv2 introduced explicit Leave Group messages so that group departure is prompt rather than waiting for a query timeout. When the router receives the Leave, it sends a Group-Specific Query to check if any other hosts on the segment still want the group. If no Membership Reports are received within the query interval, the router stops forwarding to that link. This 'fast-leave' mechanism was a major improvement over IGMPv1, where the router had to wait for multiple query cycles (potentially minutes) before realizing no host needed the group.

Question 3

IGMP is responsible for routing multicast traffic between routers across the Internet.

• T. True
• F. False

Answer: False

IGMP is strictly a local protocol — it operates only between hosts and their first-hop router on the same network segment. Its job is to tell the local router which multicast groups have interested listeners on that link. Routing multicast traffic between routers across the Internet is the job of multicast routing protocols such as PIM (Protocol Independent Multicast). IGMP provides the per-link membership information that those routing protocols use to build distribution trees, but IGMP itself never crosses a router boundary.

Question 4

In a subnet with 100 hosts most listening to the same multicast group, most 100 hosts will send IGMP Membership Reports in response to a router's Membership Query.

• T. True
• F. False

Answer: False

IGMP includes a report suppression mechanism (in IGMPv1 and v2) to prevent exactly this scenario. When a host receives a query, it starts a random timer. If it hears another host on the same segment report the same group before its timer expires, it cancels its own report — the router only needs one confirmation that the group is active on the link. This random back-off ensures that only one (or a few) reports are sent per group per query cycle, keeping IGMP overhead minimal even on densely populated segments.

Question 5

Why does the router send a Group-Specific Query after receiving an IGMPv2 Leave Group message, rather than immediately stopping multicast forwarding for that group?

Model answer: Because the Leave Group message comes from one host, but there may be other hosts on the same link still listening to that group. The router cannot safely stop forwarding based on one host's departure — doing so would cut off any remaining listeners. The Group-Specific Query asks 'does anyone else on this link still want this group?' Only if no Membership Reports are received does the router stop forwarding. This ensures correct behavior in multi-host environments while still achieving fast leave compared to waiting for a full query cycle.

This design reflects IGMP's core operating model: the router maintains per-link group state based on whether *any* host on the link wants the group, not which specific hosts do. It cannot track individual host memberships — it only needs to know if forwarding is needed at all. The Group-Specific Query is the minimum-cost mechanism to verify that forwarding can safely stop.