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Catalyst 2950 Desktop Switch Software Configuration Guide
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Chapter 17 Co nfiguring IGMP S noo ping and M VR
Unders tanding IGMP Snooping
Figure 17-2 Second Host Joining a Multicast Group

Leaving a Multicast Group

The ro uter send s periodic mu lticast genera l queri es and the sw itch forwar ds these que ries through al l
ports in the VLAN. Interested hosts respond to the queries. If at least one host in the VLAN wishes to
receive multicast traffic, the router continues forwarding the multicast traffic to the VLAN. The switch
forwards multicast group traffic to only those hosts listed in the forwarding table for that Layer 2
multicast group.
When hosts want to leave a multicast group, they can either silently leave, or they can send a leave
message. When the switch receives a leave message from a host, it sends out a MAC-based general query
to determine if any other devices connected to that interface are interested in traffic for the specific
multic ast gro up. The swi tch the n upd ate s th e for wardi ng ta ble fo r th at MAC group so th at onl y those
hosts inter ested in recei vin g multicast traf fic for the grou p are listed in the forwar ding table. If the rou ter
receives no reports fr om a VLAN, i t removes the group for th e VLAN fro m its IGMP cache.

Immediate-Leave Processing

The switc h uses IGMP snoo ping Im mediat e-Leave processin g to remove from the forwa rding tabl e an
interface tha t sends a leave messag e without th e switch sendi ng MAC-base d general queri es to the
interf ace. The VLAN inter f ac e is pru ned f rom the multicast tree for th e multi cast g rou p speci fied in the
original leave message. Immediate-Leave processing ensures optimal bandwidth management for all
hosts on a switched network , even whe n multiple mu lticast gr oups a re simultaneou sly in use .
Table 17-2 Updated IGMP Snooping Forwarding Table
Dest inat io n Addr ess Type of Packe t Ports
0100.5exx.xxxx IGMP 0
0100 .5e01 .0 203 !IGMP 1, 2, 5
Forwarding
table
CPU
Host 1 Host 2 Host 3 Host 4
Router A
Switching engine VLAN
1
0
234 5
45751