Chapter 29 IGMP

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 127 IGMP Version 1 Example

1 Query

2 Report

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 128 IGMP Version 2 Example

1 Query

2 Report

 

3 Leave

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GS-4012F/4024 User’s Guide