IGMPv3 includes the following key changes from IGMPv2:
IGMPv3 supports source-specific multicast (SSM), which builds shortest path trees from each receiver to the source, through the following features:
Host messages that can specify both the group and the source.
The multicast state that is maintained for groups and sources, not just for groups as in IGMPv2.
Hosts no longer perform report suppression, which means that hosts always send IGMP membership reports when an IGMP query message is received.
For detailed information about IGMPv2, see RFC 2236.
The basic IGMP process of a router that discovers multicast hosts is shown in Figure 1-10. Hosts 1, 2, and 3 send unsolicited IGMP membership report messages to initiate receiving multicast data for a group or channel.
Figure 1-10 IGMPv1 and IGMPv2 Query-Response Process
In Figure 1-10, router A, which is the IGMP designated querier on the subnet, sends query messages to the all-hosts multicast group at 224.0.0.1 periodically to discover whether any hosts want to receive multicast data. You can configure the group membership timeout value that the router uses to determine that no members of a group or source exist on the subnet.
The software elects a router as the IGMP querier on a subnet if it has the lowest IP address. As long as a router continues to receive query messages from a router with a lower IP address, it resets a timer that is based on its querier timeout value. If the querier timer of a router expires, it becomes the designated querier. If that router later receives a host query message from a router with a lower IP address, it drops its role as the designated querier and sets its querier timer again.
In Figure 1-10, host 1’s membership report is suppressed, and host 2 sends its membership report for group 239.0.0.1 first. Host 1 receives the report from host 2. Because only one membership report per group needs to be sent to the router, other hosts suppress their reports to reduce network traffic. Each host waits for a random time interval to avoid sending reports at the same time. You can configure the query maximum response time parameter to control the interval in which hosts randomize their responses.
Note
IGMPv2 membership report suppression occurs only on hosts that are connected to the same port.
In Figure 1-11, router A sends the IGMPv3 group-and-source-specific query to the LAN. Hosts 2 and 3 respond to the query with membership reports that indicate that they want to receive data from the advertised group and source. This IGMPv3 feature supports SSM.
Figure 1-11 IGMPv3 Group-and-Source-Specific Query
Note
IGMPv3 hosts do not perform IGMP membership report suppression.
IGMP messages sent by the designated querier have a time-to-live (TTL) value of 1, which means that the messages are not forwarded by the directly connected routers on the subnet. You can configure the frequency and number of query messages sent specifically for IGMP start-up, and you can configure a short query interval at start-up so that the group state is established as quickly as possible. Although usually unnecessary, you can tune the query interval used after start-up to a value that balances the responsiveness to host group membership messages and the traffic created on the network.
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