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Catalyst 6500 Series Switch Software Configuration Guide—Release 8.7
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Chapter 51 Configuring Multicast Services
Understanding How Multicasting Works
Leaving a Multicast Group
In a network running IGMP version 1 or 2, the designated multicast router for a segment continues forwarding
the multicast traffic to that VLAN as long as at least one host in the VLAN wishes to receive the multicast
traffic. When the hosts want to leave a multicast group, they can either ignore the periodic general queries
that are sent by the multicast router (IGMP version 1 host behavior), or they can send an IGMP leave (IGMP
version 2 host behavior). In the systems with a Supervisor Engine 1 or 2, when the switch receives a leave
message, it sends out a MAC-based general query on the port on which it received the leave message to
determine if any of the devices that are connected to this port are interested in the traffic for the specific
multicast group. If this port is the last port in the VLAN, the switch sends a MAC-based general query to all
the ports in the VLAN. The MAC-based general queries are addressed to the Layer 2 Group Destination
Address (GDA) MAC address for which the IGMP leave message was received. At Layer 3, the
MAC-based general queries are addressed to 244.0.0.1 (all hosts), and in the IGMP header, the group
address field is set to 0.0.0.0.
If no IGMP join is received for any of the IP multicast groups that map to the MAC multicast group address,
the port is removed from the multicast forwarding entry. If the port is not the last nonmulticast-router
port in the entry, the switch suppresses the IGMP leave (does not send it to the router). If the port is the last
nonmulticast-router port in the entry, the IGMP leave is forwarded to the multicast router ports and the
MAC group forwarding entry is removed.
When the router receives the IGMP leave, it sends several IGMP group-specific queries. If no join
messages are received in response to the queries, and there are no downstream routers that are connected
through that interface, the router removes the interface from the OIL for that IP multicast group entry in
the multicast routing table.
IGMP Fast-Leave Processing
IGMP snooping fast-leave processing allows the switch processor to remove an interface from the port
list of a forwarding-table entry without first sending out a MAC-based general query on the port. When an
IGMP leave is received on a port, the port is immediately removed from the multicast forwarding entry (or
the entire entry is removed).
IGMP Fast-Block Processing
IGMP version 3 supports fast-block processing. If you enable fast-block processing on the switch, the
switch immediately stops forwarding the multicast packets to a port when it receives a block or exclude
message from a host connected to that port.
Note
Do not use the fast-leave processing feature if more than one host is connected to each port. If you enable
fast-leave when more than one host is connected to a port, some hosts might be dropped inadvertently.
Fast leave is supported with IGMP version 2 hosts only.