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Chapter 20
Configuring IGMP Snooping and MVR
Understanding IGMP Snooping
Note
IGMPv3 join and leave messages are not supported on switches running IGMP filtering or MVR.
An IGMPv3 switch can receive messages from and forward messages to a device running the Source
Specific Multicast (SSM) feature. For more information about source-specific multicast with IGMPv3
and IGMP, see this URL:
http://www.cisco.com/univercd/cc/td/doc/product/software/ios121/121newft/121t/121t5/dtssm5t.htm
Joining a Multicast Group
When a host connected to the switch wants to join an IP multicast group and it is an IGMP Version 2
client, it sends an unsolicited IGMP join message, specifying the IP multicast group to join.
Alternatively, when the switch receives a general query from the router, it forwards the query to all ports
in the VLAN. IGMP Version 1 or Version 2 hosts wanting to join the multicast group respond by sending
a join message to the switch. The switch CPU creates a multicast forwarding-table entry for the group if
it is not already present. The CPU also adds the interface where the join message was received to the
forwarding-table entry. The host associated with that interface receives multicast traffic for that
multicast group. See
Figure
20-1.
Figure 20-1
Initial IGMP Join Message
Router A
1
IGMP report 224.1.2.3
VLAN
PFC
CPU
0
Forwarding
table
2
3
4
5
Host 1
Host 2
Host 3
Host 4
Router A sends a general query to the switch, which forwards the query to ports 2 through 5, which are
all members of the same VLAN. Host 1 wants to join multicast group 224.1.2.3 and multicasts an IGMP
membership report (IGMP join message) to the group. The switch CPU uses the information in the IGMP
report to set up a forwarding-table entry, as shown in
Table
20-1, that includes the port numbers
connected to Host 1 and the router.
Cisco ME 3400 Ethernet Access Switch Software Configuration Guide
20-3
78-17058-01
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Troubleshooting
