Cisco 7604 Configuration Manual page 466
Ios software configuration guide
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Understanding How IGMP Snooping Works
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IGMP Version 3 Support Overview
IGMP snooping supports IGMP version 3. IGMP version 3 uses source-based filtering, which enables
hosts and routers to specify which source addresses should be allowed or blocked for a specific multicast
group. When you enable IGMP version 3 snooping on a Cisco 7600 series router, the system maintains
IGMP version 3 states based on messages it receives for a particular group in a particular VLAN and
either allows or blocks traffic based on the following information in these messages:
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Because the Layer 2 table is (MAC-group, VLAN) based, with IGMPv3 hosts it is preferable to have
only a single multicast source per MAC-group.
Source-based filtering for IGMP version 3 reports is not supported in hardware. The states are
Note
maintained only in software and used for explicit host tracking and statistics collection. The source-only
entries are deleted every 5 minutes and relearned to ensure that they are still valid.
IGMPv3 Fast-Leave Processing
IGMP version 3 fast-leave processing is enabled by default. To disable IGMP version 3 fast-leave
processing you must turn off explicit-host tracking.
Fast-leave processing with IGMPv3 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 router 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 router 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 router does not remove the host from the LTL index until the host is
no longer interested in receiving traffic from any source.
Proxy Reporting
IGMP supports proxy reporting for IGMPv1 and IGMPv2 messages to handle group-specific queries. These
queries are not sent downstream, but the switch does respond to them directly. When the switch recieves a
group-specific query, the switch terminates the query and sends an IGMP proxy report if there is a receiver
for the group. There is no proxy reporting for IGMPv3 messages. For IGMPv3, a group-specific query or a
group source-specific query is flooded to all VLAN member ports. The database for the IGMPv3 membership
report is built based on the reports received.
Host reports responding to a specific query can be suppressed by the report suppression feature. Report
suppression is supported for IGMPv1, IGMPv2 and IGMPv3 messages. With report suppression enabled (by
default), when the switch recieves a general query, the switch starts a suppression cycle for reports from all
hosts to each group or channel (S,G). Only the first report to the discovered multicast routers are forwarded;
the rest of the reports are suppressed. For IGMPv1 and IGMPv2, the time of suppression is the report response
time indicated in the general query message. For IGMPv3, suppression occurs for the entire general query
interval.
Cisco 7600 Series Router Cisco IOS Software Configuration Guide, Release 12.2SX
30-6
Explicit Host Tracking, page 30-7
Source lists
Allow (include) or block (exclude) filtering options
Chapter 30
Configuring IGMP Snooping for IPv4 Multicast Traffic
OL-4266-08
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