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Understanding IGMP Snooping
Figure 22-1
Forwarding
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. Blade Server 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
numbers of Blade Server 1 and the router.
Table 22-1
Destination Address
224.1.2.3
The switch hardware can distinguish IGMP information packets from other packets for the multicast
group. The information in the table tells the switching engine to send frames addressed to the 224.1.2.3
multicast IP address that are not IGMP packets to the router and to the host that has joined the group.
If another blade server (for example, Blade Server 4) sends an unsolicited IGMP join message for the
same group
the forwarding table as shown in
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.
Cisco Catalyst Blade Switch 3020 for HP Software Configuration Guide
22-4
Initial IGMP Join Message
Switching engine
CPU
0
table
1
3
Blade
Blade
Server 1
Server 2
IGMP Snooping Forwarding Table
Type of Packet
IGMP
(Figure
22-2), the CPU receives that message and adds the port number of Blade Server 4 to
Router A
19
IGMP report 224.1.2.3
VLAN
5
7
Blade
Blade
Server 3
Server 4
Ports
1, 2
Table
22-2. Note that because the forwarding table directs IGMP
Chapter 22
Configuring IGMP Snooping and MVR
Table
22-1, that includes the port
OL-8915-03
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Troubleshooting
