Cisco Systems 3750E manual 224.1.2.3, 24-4

Chapter 24 Configuring IGMP Snooping and MVR

Understanding IGMP Snooping

Figure 24-1 Initial IGMP Join Message

Router A

CPU 0

1

IGMP report 224.1.2.3

VLAN

PFC

Forwarding

table

2

3

4

45750

5

Host 1 Host 2 Host 3 Host 4

Router A sends a general query to the switch, which forwards the query to ports 2 through 5, which are 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. The switch CPU uses the information in the IGMP report to set up a forwarding-table entry, as shown in Table 24-1, that includes the port numbers connected to Host 1 and the router.

Table 24-1	IGMP Snooping Forwarding Table
Destination Address		Type of Packet	Ports
224.1.2.3		IGMP	1, 2

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.

Catalyst 3750-E and 3560-E Switch Software Configuration Guide

24-4 OL-9775-02