Installation guide

Network Infrastructure for EtherNet/IP™
Designing the Infrastructure
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Figure 4-8 Second Host Joining a Multicast Group.
If another host (for example, Host 4 in Figure 4-8) sends an IGMP join message for the same group, the
CPU receives that message and adds the port number of Host 4 to the multicast forwarding table as shown
in Table 4-12.
Table 4-12 Updated Multicast Forwarding Table Entry
Updated Multicast Forwarding Table Destination Address Type of Packet Ports
0100.5E01.0203 !IGMP 1, 2, 5
4.8.2 Leaving a Multicast Group
The router sends periodic IP multicast general queries, and the switch responds to these queries with one
join response per MAC multicast group. As long as at least one host in the VLAN needs multicast traffic,
the switch responds to the router queries, and the router continues to forward the multicast traffic to the
VLAN. The switch only forwards IP multicast group traffic to those hosts listed in the forwarding table
for that IP multicast group.
When hosts need to leave a multicast group, they can either ignore the periodic general-query requests
sent by the router, or they can send a leave message. When the switch receives a leave message from a
host, it sends out a group-specific query to determine if any devices behind that interface are interested in
traffic for the specific multicast group. If, after a number of queries, the router processor receives no
reports from a VLAN, it removes the group for the VLAN from its multicast-forwarding table.
4.8.3 Immediate-Leave Processing
IGMP snooping immediate-leave processing allows the switch to remove an interface that sends a leave
message from the forwarding table without first sending out MAC-based general queries to the interface.
The VLAN interface is pruned from the multicast tree for the multicast group specified in the original