Chapter 37 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.

37.1.1 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 172 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

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