Installation guide

Network Infrastructure for EtherNet/IP™
Designing the Infrastructure
4-47
4.8.1 Joining a Multicast Group
When a host connected to the switch wants to join an IP multicast group, it sends an IGMP join request,
specifying the IP multicast group it wants to join. When the switch receives this message, it adds the port
to the IP multicast group port address entry in the forwarding table.
In Figure 4-7, Host 1 wants to join multicast group 224.1.2.3 and multicasts an unsolicited IGMP Join
Report (IGMP join message) to the group with the equivalent MAC destination address of
0100.5E01.0203. The switch recognizes IGMP packets and forwards them to the CPU.
Figure 4-7 Initial IGMP Join Message.
When the CPU receives the IGMP report multicast by Host 1, the CPU uses the information to set up a
multicast forwarding table entry as shown in Table 4-11. This entry includes the port numbers of Host 1
and the router.
Table 4-11 Multicast Forwarding Table Entry
IP Multicast Forwarding Table Destination Address Type of Packet Ports
0100.5E01.0203 !IGMP 1, 2
The switch architecture allows the CPU to distinguish IGMP information packets from other packets for
the multicast group. The switch recognizes the IGMP packets through its filter engine. This prevents the
CPU from becoming overloaded with multicast frames.
The entry in the multicast-forwarding table tells the switching engine to send frames addressed to the
0100.5E01.0203 multicast MAC address that are not IGMP packets (!IGMP) to the router and to the host
that has joined the group.