Configuring IP Multicast
What is Internet Group Management Protocol – IGMP?

Internet Group Management Protocol (IGMP) is used to register individual hosts with a multicast group. The host sends a join message to a local router multicast address. If the router is running a multicast routing protocol, it will accept the join and then forward the multicast stream for that group onto the segment where the registering host is present. IGMP messages are IP datagrams with a protocol value of 2 and a destination address 224.0.0.2 and a TTL of 1.

In addition to listening to IGMP join messages, multicast routers also periodically send out queries to discover which groups are active or inactive on a particular subnet. Any end station that is part of the multicast group receives this IGMP query, and responds with a host membership report for each group to which it belongs . This is sent to all hosts 224.0.0.1 with a TTL of 1.

As of this writing, version 3 is the most current iteration of IGMP and is covered in more detail. Previous versions had attributes and limitations as listed in Figure .

IGMP Message Format
IGMP version3, the next step in the evolution of IGMP, adds support for source filtering, multiple group memberships, joins and leaves. This enables a multicast receiving host to indicate to the router the groups from which it wants to receive multicast traffic, as well as the source unicast addresses from which this traffic is expected. This membership information enables IOS software to forward traffic from only those sources requested by the receiver. IGMP v3 supports Report and Query messages that have different packet structure as shown .

IGMP v3 Report Message
Figure describes the fields in the IGMP v3 report message sent from the host to the router.

With IGMP v3, receivers signal membership to a multicast host group in these two modes:

  • INCLUDE mode – The receiver announces membership to a host group and provides a list of source addresses (the INCLUDE list) from which it does want to receive traffic.
  • EXCLUDE mode – The receiver announces membership to a multicast group and provides a list of source addresses (the EXCLUDE list) from which it does not want to receive traffic. To receive traffic from all sources, which is the behavior of IGMP v2, a host uses EXCLUDE mode membership with an empty EXCLUDE list.

IGMP v3 Query Message
The IGMP query message sent from the multicast router to the all hosts address 224.0.0.1 has a different format than the report or join message.  –