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23-5
Cisco Catalyst Switch Module 3110 and 3012 for IBM BladeCenter Software Configuration Guide
OL-12189-01
Chapter 23 Configuring IGMP Snooping and MVR
Understanding IGMP Snooping
If another blade server (for example, Blade Server 4) sends an unsolicited IGMP join message for the
same group (Figure 23-2), the CPU receives that message and adds the port number of Blade Server 4 to
the forwarding table as shown in Table 23-2. Note that because the forwarding table directs IGMP
messages only to the CPU, the message is not flooded to other ports on the switch. Any known multicast
traffic is forwarded to the group and not to the CPU.
Figure 23-2 Second Host Joining a Multicast Group
Leaving a Multicast Group
The router sends periodic multicast general queries, and the switch forwards these queries through all
ports in the VLAN. Interested blade servers respond to the queries. If at least one blade server in the
VLAN wishes to receive multicast traffic, the router continues forwarding the multicast traffic to the
VLAN. The switch forwards multicast group traffic only to those blade servers listed in the forwarding
table for that IP multicast group maintained by IGMP snooping.
When blade servers want to leave a multicast group, they can silently leave, or they can send a leave
message. When the switch receives a leave message from a blade server, it sends a group-specific query
to learn if any other devices connected to that interface are interested in traffic for the specific multicast
group. The switch then updates the forwarding table for that MAC group so that only those blade servers
interested in receiving multicast traffic for the group are listed in the forwarding table. If the router
receives no reports from a VLAN, it removes the group for the VLAN from its IGMP cache.
Table 23-2 Updated IGMP Snooping Forwarding Table
Destination Address Type of Packet Ports
224.1.2.3 IGMP 19, 1, 7
Forwarding
table
CPU
Router A
Switching engine
VLAN
19
0
135 7
201773
Blade
Server 1
Blade
Server 2
Blade
Server 3
Blade
Server 4