R21xx-HP FlexFabric 11900 IP Multicast Configuration Guide

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IGMPv1 does not define a leave group message (often called a "leave message"). When an IGMPv1
host is leaving a multicast group, it stops sending reports to that multicast group. If the subnet has no
members for a multicast group, the IGMP routers will not receive any report addressed to that multicast
group. In this case, the routers clear the information for that multicast group after a period of time.
IGMPv2 enhancements
Backwards-compatible with IGMPv1, IGMPv2 has introduced a querier election mechanism and a
leave-group mechanism.
Querier election mechanism
In IGMPv1, the DR elected by the Layer 3 multicast routing protocol (such as PIM) serves as the querier
among multiple routers that run IGMP on the same subnet.
IGMPv2 introduced an independent querier election mechanism. The querier election process is as
follows:
1. Initially, every IGMPv2 router assumes itself to be the querier and sends IGMP general query
messages (often called "general queries") to all hosts and routers on the local subnet. The
destination address is 224.0.0.1.
2. After receiving a general query, every IGMPv2 router compares the source IP address of the query
message with its own interface address. After comparison, the router with the lowest IP address
wins the querier election and all the other IGMPv2 routers become non-queriers.
3. All the non-queriers start a timer, known as an "other querier present timer." If a router receives an
IGMP query from the querier before the timer expires, it resets this timer. Otherwise, it considers
the querier to have timed out and initiates a new querier election process.
"Leave group" mechanism
In IGMPv1, when a host leaves a multicast group, it does not send any notification to the multicast routers.
The multicast routers determine whether a group has members by using the maximum response delay.
This adds to the leave latency.
In IGMPv2, when a host leaves a multicast group, the following process occurs:
1. The host sends a leave message to all routers on the local subnet. The destination address is
224.0.0.2.
2. After receiving the leave message, the querier sends a configurable number of group-specific
queries to the group that the host is leaving. Both the destination address field and the group
address field of the message are the address of the multicast group that is being queried.
3. One of the remaining members (if any on the subnet) of the group should send a membership
report within the maximum response delay advertised in the query messages.
4. If the querier receives a membership report for the group before the maximum response delay
timer expires, it maintains the memberships for the group. Otherwise, the querier assumes that the