Cisco Catalyst 2950 Software Configuration Manual page 367

Chapter 16 Configuring IGMP Snooping and MVR
Figure 16-1 Initial IGMP Join Message
Table 16-1 IP Multicast Forwarding Table
Destination Address
0100.5e01.0203
Note that the switch architecture allows the CPU to distinguish IGMP information packets from other
packets for the multicast group. The switch recognizes the IGMP packets through its filter engine. This
prevents the CPU from becoming overloaded with multicast frames.
The entry in the multicast forwarding table tells the switching engine to send frames addressed to the
0100.5E01.0203 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 IGMP join message for the same group
CPU receives that message and adds the port number of Host 4 to the multicast forwarding table as
shown in
78-11380-04
Catalyst 2950 switch
CPU
0
CAM
Table
2 3
Host 1 Host 2
Type of Packet
!IGMP
Table 16-2.
Router A
1
IGMP Report 224.1.2.3
4 5
Host 3 Host 4
Ports
1, 2
Catalyst 2950 Desktop Switch Software Configuration Guide
Understanding IGMP Snooping
(Figure 16-2), the
16-3