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Catalyst 3550 Multilayer Switch Software Configuration Guide
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Chapter11 Configuring IGMP Snooping and M VR Understanding IGMP Snooping
Figure11-1 Initial IGMP Join Message
Router A sends a general query to the switch, which forwards the query to ports 2 through 5, all membe rs
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 with the equivalent MAC desti nat ion ad dress of
0x0100.5E01.0203. When the CPU receives the IGMP report mu lticast by H ost 1 , the CPU us es the
information in the IGMP report to set up a forwarding-table entry, as shown in Table 11-1, that includes
the port numbers of Host 1, the router, and the switch internal CPU.
Note that the switch hardware can distinguish IGMP information packets from other packet s for the
multicast group.
•The first entry in the table tells the switching engine to send IGMP packets to only the swi tch CPU.
This prevents the CPU from becoming overloaded with multicast frames.
•The second entry tells the switching engine to send frames addressed to the 0x0100.5E01.0203
multicast MAC address that are not IGMP packets (!IGMP) to the router and to the host that has
joined the group.
If another host (for example, Host 4) sends an unsolicited IGMP join m e ssag e for the sam e group
(Figure 11-2), the CPU receives that message and adds the port number of H ost 4 to the fo rwardin g table
as shown in Table 11-2. Note that because the forwarding table directs IGMP messages to only the CPU,
the message is not flooded to other ports on the switch. Any multica st tra ffic is for ward ed to t he gr oup
and not to the CPU.
Forwarding
table
CPU
Host 1 Host 2 Host 3 Host 4
Router A
IGMP report 224.1.2.3
VLAN
Switching engine
1
0
2345
45750
Table11-1 IGMP Snooping Forwarding Table
Destination Address Type of Packet Ports
0100.5exx.xxxx IGMP 0
0100.5e01.0203 !IGMP 1, 2