Leaving A Multicast Group - Cisco WS-SUP32-GE-3B - Supervisor Engine 32 Software Configuration Manual

Understanding How MLDv2 Snooping Works

Leaving a Multicast Group

These sections describe leaving a multicast group:
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Normal Leave Processing
Interested hosts must continue to respond to the periodic MLDv2 general queries. As long as at least one
host in the VLAN responds to the periodic MLDv2 general queries, the multicast router continues
forwarding the multicast traffic to the VLAN. When hosts want to leave a multicast group, they can
either ignore the periodic MLDv2 general queries (called a "silent leave"), or they can send an MLDv2
filter mode change record.
When MLDv2 snooping receives a filter mode change record from a host that configures the EXCLUDE
mode for a group, MLDv2 snooping sends out a MAC-addressed general query to determine if any other
hosts connected to that interface are interested in traffic for the specified multicast group.
If MLDv2 snooping does not receive an MLDv2 report in response to the general query, MLDv2
snooping assumes that no other hosts connected to the interface are interested in receiving traffic for the
specified multicast group, and MLDv2 snooping removes the interface from its Layer 2 forwarding table
entry for the specified multicast group.
If the filter mode change record was from the only remaining interface with hosts interested in the group,
and MLDv2 snooping does not receive an MLDv2 report in response to the general query, MLDv2
snooping removes the group entry and relays the MLDv2 filter mode change record to the multicast
router. If the multicast router receives no reports from a VLAN, the multicast router removes the group
for the VLAN from its MLDv2 cache.
The interval for which the switch waits before updating the table entry is called the "last member query
interval." To configure the interval, enter the ipv6 mld snooping last-member-query-interval interval
command.
Fast-Leave Processing
Fast-leave processing is enabled by default. To disable fast-leave processing, turn off explicit-host
tracking.
Fast-leave processing is implemented by maintaining source-group based membership information in
software while also allocating LTL indexes on a MAC GDA basis.
When fast-leave processing is enabled, hosts send BLOCK_OLD_SOURCES{src-list} messages for a
specific group when they no longer want to receive traffic from that source. When the switch receives
such a message from a host, it parses the list of sources for that host for the given group. If this source list
is exactly the same as the source list received in the leave message, the switch removes the host from the LTL
index and stops forwarding this multicast group traffic to this host.
If the source lists do not match, the switch does not remove the host from the LTL index until the host
is no longer interested in receiving traffic from any source.
Note Disabling explicit host tracking disables fast-leave processing and proxy reporting.
Catalyst Supervisor Engine 32 PISA Cisco IOS Software Configuration Guide, Release 12.2ZY
26-6
Normal Leave Processing, page 26-6
Fast-Leave Processing, page 26-6
Chapter 26 Configuring MLDv2 Snooping for IPv6 Multicast Traffic
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