Joining A Multicast Group - Cisco Catalyst 3550 series Software Configuration Manual

Chapter 20 Configuring IGMP Snooping and MVR
An IGMPv3 switch can receive messages from and forward messages to a device running the Source
Specific Multicast (SSM) feature. For more information, refer to the "Configuring IP Multicast Layer 3
Switching" chapter in the Catalyst 4500 Series Switch Cisco IOS Software Configuration Guide, Cisco
IOS Release 12.1(12c)EW at this URL:
http://www.cisco.com/univercd/cc/td/doc/product/lan/cat4000/12_1_12/config/mcastmls.htm

Joining a Multicast Group

When a host connected to the switch wants to join an IP multicast group, it sends an unsolicited IGMP
join message, specifying the IP multicast group to join. Alternatively, when the switch receives a general
query from the router, it forwards the query to all ports in the VLAN. Hosts wanting to join the multicast
group respond by sending a join message to the switch. The switch CPU creates a multicast
forwarding-table entry for 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 host associated with that interface
receives multicast traffic for that multicast group. See
Figure 20-1 Initial IGMP Join Message
Forwarding
Router A sends a general query to the switch, which forwards the query to ports 2 through 5, all members
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 destination address of
0x0100.5E01.0203. When the CPU receives the IGMP report multicast by Host 1, the CPU uses the
information in the IGMP report to set up a forwarding-table entry, as shown in
the port numbers of Host 1, the router, and the switch internal CPU.
Table 20-1 IGMP Snooping Forwarding Table
Destination Address
0100.5exx.xxxx
0100.5e01.0203
78-11194-09
Switching engine
CPU
0
table
2 3
Host 1 Host 2
Type of Packet
IGMP
!IGMP
Figure
Router A
1
IGMP report 224.1.2.3
VLAN
4 5
Host 3 Host 4
Ports
0
1, 2
Catalyst 3550 Multilayer Switch Software Configuration Guide
Understanding IGMP Snooping
20-1.
Table 20-1, that includes
20-3