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In the case of IGMPv1 or IGMPv2, Host B cannot select multicast sources when it joins multicast group 

G. Therefore, multicast streams from both Source 1 and Source 2 will flow to Host B whether it needs 

them or not.  

When IGMPv3 is running between the hosts and routers, Host B can explicitly express its interest in the 

multicast data Source 1 sends to multicast group G (denoted as (S1, G)), rather than the multicast data 

Source 2 sends to multicast group G (denoted as (S2, G)). Thus, only multicast data from Source 1 will 

be delivered to Host B.  

1)  Query message carrying the source addresses  

IGMPv3 supports not only general queries (feature of IGMPv1) and group-specific queries (feature of 

IGMPv2), but also group-and-source-specific queries.  

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A general query does not carry a group address, nor a source address;  

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A group-specific query carries a group address, but no source address;  

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A group-and-source-specific query carries a group address and one or more source addresses.  

2)  Reports containing multiple group records  

Unlike an IGMPv1 or IGMPv2 report message, an IGMPv3 report message is destined to 224.0.0.22 

and contains one or more group records. Each group record contains a multicast group address and a 

multicast source address list. 

Group record types include:  

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IS_IN: The source filtering mode is Include, namely, the report sender requests the multicast data 

from only the sources defined in the specified multicast source list. 

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IS_EX: The source filtering mode is Exclude, namely, the report sender requests the multicast data 

from any sources but those defined in the specified multicast source list.  

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TO_IN: The filtering mode has changed from Exclude to Include. 

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TO_EX: The filtering mode has changed from Include to Exclude. 

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ALLOW: The Source Address fields in this Group Record contain a list of the additional sources 

that the system wishes to hear from, for packets sent to the specified multicast address. If the 

change was to an Include source list, these are the addresses that were added to the list; if the 

change was to an Exclude source list, these are the addresses that were deleted from the list. 

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BLOCK: indicates that the Source Address fields in this Group Record contain a list of the sources 

that the system no longer wishes to hear from, for packets sent to the specified multicast address. 

If the change was to an Include source list, these are the addresses that were deleted from the list; 

if the change was to an Exclude source list, these are the addresses that were added to the list. 

The IGMP SSM mapping feature allows you to configure static IGMP SSM mappings on the last hop 

router to provide SSM support for receiver hosts running IGMPv1 or IGMPv2. The SSM model assumes 

that the last hop router is aware of the desired multicast sources when receivers join multicast groups.  

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When a host running IGMPv3 joins a multicast group, it can explicitly specify one or more multicast 

sources in its IGMPv3 report.  

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A host running IGMPv1 or IGMPv2, however, cannot specify multicast source addresses in its 

report. In this case, you need to configure the IGMP SSM mapping feature to translate the (*, G) 

information in the IGMPv1 or IGMPv2 report into (G, INCLUDE, (S1, S2...)) information.