Chapter 51 IGMP
XGS4600 Series User’s Guide
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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.
51.1.1 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 372 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.
Figure 373 IGMP Version 2 Example
IGMP version 3 allows a multicast host to join a multicast group and specify from which source (multicast
server) it wants to receive multicast packets. Alternatively, a multicast host can specify from which
multicast servers it does not want to receive multicast packets. In the following figure multicast server X
(IP address 10.1.1.1) and multicast server Z (IP address 13.2.2.2) both send multicast traffic to the same
multicast group identified by the multicast IP address 225.1.1.1. In IGMP version 3 multicast host A can
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