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Understanding IGMP Snooping
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 blade server associated with that interface receives multicast
traffic for that multicast group. See
Figure 24-1
Forwarding
Router A sends a general query to the switch, which forwards the query to ports 3,5, and 7, 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 24-1
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.
Cisco Catalyst Blade Switch 3130 and 3032 for Dell Software Configuration Guide
24-4
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
24-1.
Router A
19
IGMP report 224.1.2.3
VLAN
5
7
Blade
Blade
Server 3
Server 4
Ports
19, 1
Chapter 24
Configuring IGMP Snooping and MVR
Table
24-1, that includes the port
OL-12247-04
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