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Catalyst2950 and Catalyst2955 Switch Software Configuration Guide
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Chapter21 Configuring IGMP Snooping and M VR Understanding IGMP Snooping
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
multicast group. The switch then updates the forwarding ta ble fo r th at MAC group so th at onl y those
hosts interested in receiving multicast traffic for the group are listed in the forw arding tab le. If the rou ter
receives no reports from a VLAN, it removes the group for the VLAN from its IGMP cache.
Immediate-Leave Processing
Immediate Leave is only supported with IGMP version2 hosts.
The switch uses IGMP snooping Immediate-Leave processing to remove from the forwarding table an
interface that sends a leave message without the switch sending MAC-based general queries to the
interface. The VLAN interface is pruned from the multicast tree for the multicast group specified in the
original leave message. Immediate-Leave processing ensures optimal b and wid th m an agem e nt f or all
hosts on a switched network, even when multiple multicast groups are simultaneously in use.
Note You should only use the Immediate-Leave processing feature on VLANs where a si ng le h ost i s
connected to each port. If Immediate Leave is enabled in VLANs where mor e than one ho st is connected
to a port, some hosts might inadvertently be dropped.
IGMP Report Suppression
Note IGMP report suppression is supported only when the multicast query has IGMPv1 and IGMPv2 reports.
This feature is not supported when the query includes IGMPv 3 r eport s.
The switch uses IGMP report suppression to forward only one IGMP report per multicast router query
to multicast devices. When IGMP router suppression is enabled (the default), the switch sends the first
IGMP report from all hosts for a group to all the multicast routers. The switch does not send the
remaining IGMP reports for the group to the multicast routers. This feature prevents duplicate reports
from being sent to the multicast devices.
If the multicast router query includes requests only for IGMPv1 and IGMPv2 reports, the switch
forwards only the first IGMPv1 or IGMPv2 report from all hosts for a group to all the multicast routers.
If the multicast router query also includes requests for IGMPv3 reports, the switch forwards all IGMPv1,
IGMPv2, and IGMPv3 reports for a group to the multicast devices .
If you disable IGMP report suppression, all IGMP reports are forwarded to the multicast routers.
Source-Only Networks
In a source-only network, switch ports are connected to multicast source ports and multicast router ports.
The switch ports are not connected to hosts that send IGMP join or leave messages.
The switch learns about IP multicast groups from the IP multicast data stre a m by using the source-only
learning method. The switch forwards traffic only to the multicast router ports.