Cisco NM-8B-U - HW ROUTERS L-M User Manual page 37

16- and 36-Port Ethernet Switch Module for Cisco 2600 Series, Cisco 3600 Series, and Cisco 3700 Series
Joining a Multicast Group
When a host connected to the switch wants to join an IP multicast group, it sends an IGMP join message,
specifying the IP multicast group it wants to join. When the switch receives this message, it adds the port
to the IP multicast group port address entry in the forwarding table.
Refer to
membership report (IGMP join message) to the group with the equivalent MAC destination address of
0100.5E01.0203. The switch recognizes IGMP packets and forwards them to the CPU. When the CPU
receives the IGMP report multicast by Host 1, the CPU uses the information to set up a multicast
forwarding table entry as shown in
Figure 16
CPU port
Multicast
Forwarding
Table
Table 7
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
Figure 16. Host 1 wants to join multicast group 224.1.2.3 and multicasts an unsolicited IGMP
Initial IGMP Join Message
Cisco router with Ethernet
switch network module
2 3
Host 1 Host 2 Host 3
IP Multicast Forwarding Table
Type of Packet
!IGMP
Table 8.
Table 7 that includes the port numbers of Host 1 and the router.
1
IGMP Report 224.1.2.3
4 5
Host 4
Ports
1, 2
Cisco IOS Release 12.2(2)XT, 12.2(8)T, and 12.2(15)ZJ
Feature Overview
(Figure 17), the
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