IGMP Usage

-6 IGMP Usage
3-6.1 Background (informative)
The Internet Group Management Protocol (IGMP) is a standard protocol used by hosts to
report their IP multicast group memberships and must be implemented by any host that wishes
to receive IP multicast datagrams. IGMP messages are used by multicast routers to learn which
multicast groups have members on their attached networks. IGMP messages are also used by
switches capable of supporting “IGMP snooping” whereby the switch listens to IGMP
messages and only sends the multicast packets to ports that have joined the multicast group.
There are two versions of IGMP:
• IGMP V1 is defined in RFC1112. It defines two messages: Host Membership Query and
Host Membership Report (commonly referred to as a “join”)
• IGMP V2 is defined in RFC2236. It defines additional messages and behavior, notably the
Leave Group message.
IGMP V2 is backward compatible with V1. RFC2236 discusses the interaction between IGMP
V1 and V2 hosts and routers.
Since EtherNet/IP devices make extensive use of IP multicast for CIP transport class 0 and 1
connections, consistent IGMP usage by EtherNet/IP devices is essential in order to create wellfunctioning EtherNet/IP application networks.
3-6.2 IGMP Membership Report Messages
EtherNet/IP devices shall issue a Membership Report message when opening a CIP connection
on which they will receive multicast packets. Specifically, devices shall adhere to the following
behavior:

1. When the T->O Connection Type is multicast (originator is multicast consumer), the
   originator shall issue a Membership Report upon receipt of a successful
   Forward_Open_reply. The Membership Report shall include the IP multicast address as
   communicated in the Forward_Open_reply.
2. When the O->T Connection Type is multicast (target is multicast consumer), the target
   shall issue a Membership Report upon sending a successful Forward_Open_reply. The
   Membership Report shall include the IP multicast address as communicated in the
   Forward_Open.
   If the device has already issued a Membership Report for the IP multicast address (e.g., if the
   multicast address is being used with an existing connection) the device may, but is not required
   to, issue another Membership Report.
   Devices shall also send Membership Report messages in response to Membership Query
   messages, per the IGMP RFCs