How Igmp Works; Figure 128 Igmp Version 1 Example - ZyXEL Communications XGS-4528F User Manual

Chapter 29 IGMP
IGMP (Internet Group Management Protocol) is used by multicast hosts to indicate their
multicast group membership to multicast routers. Multicast routers can also use IGMP to
periodically check if multicast hosts still want to receive transmission from a multicast server.
In other words, multicast routers check if any hosts on their network are still members of a
specific multicast group.
The Switch supports IGMP version 1 (IGMP-v1), version 2 (IGMP-v2) and IGMP version 3
(IGMP-v3). Refer to RFC 1112, RFC 2236 and RFC 3376 for information on IGMP versions
1, 2 and 3 respectively. At start up, the Switch queries all directly connected networks to
gather group membership. After that, the Switch periodically updates this information.

29.1.1 How IGMP Works

This section describes how IGMP works and the changes it has gone through from version 1 to
version 3. IGMP version 1 defines how a multicast router checks to see if any multicast hosts
are part of a multicast group. It checks for group membership by sending out an IGMP Query
packet. Hosts that are members of a multicast group reply with an IGMP Report packet. This
is also referred to as a join group request. The multicast router then keeps a list of all networks
that have members of this multicast group and forwards multicast traffic to that network.

Figure 128 IGMP Version 1 Example

The main difference in IGMP version 2 is that it provides a mechanism for a multicast group
member to notify a multicast router that it is leaving a multicast group. The multicast router
then sends a group-specific IGMP query to check if there are any members remaining in that
group. If the multicast router does not receive an IGMP report from any members, it stops
sending multicast traffic to that group. This change helps shorten the leave convergence time,
in other words, the amount of time that a multicast router believes that there are group
members on a particular network. This in turn helps reduce the amount of multicast traffic
going through the multicast router.
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1 Query
2 Report
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