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Chapter 22
Configuring IGMP Snooping and MVR
Figure 22-2
Table 22-2
Destination Address
224.1.2.3
Leaving a Multicast Group
The router sends periodic multicast general queries, and the switch forwards these queries through all
ports in the VLAN. Interested blade servers respond to the queries. If at least one blade server in the
VLAN wishes to receive multicast traffic, the router continues forwarding the multicast traffic to the
VLAN. The switch forwards multicast group traffic only to those blade servers listed in the forwarding
table for that IP multicast group maintained by IGMP snooping.
When blade servers want to leave a multicast group, they can silently leave, or they can send a leave
message. When the switch receives a leave message from a blade server, it sends a group-specific query
to learn if any other devices connected to that interface are interested in traffic for the specific multicast
group. The switch then updates the forwarding table for that MAC group so that only those blade servers
interested in receiving multicast traffic for the group are listed in the forwarding table. If the router
receives no reports from a VLAN, it removes the group for the VLAN from its IGMP cache.
OL-8915-03
Second Host Joining a Multicast Group
Switching engine
CPU
0
Forwarding
table
1
3
Blade
Blade
Server 1
Server 2
Updated IGMP Snooping Forwarding Table
Router A
19
VLAN
5
7
Blade
Blade
Server 3
Server 4
Type of Packet
Ports
IGMP
1, 2, 5
Cisco Catalyst Blade Switch 3020 for HP Software Configuration Guide
Understanding IGMP Snooping
22-5
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