Igmpv2 Enhancements - HP 5920 series Configuration Manual

As shown in
group G1, and Host A is interested in the multicast data addressed to G2. The following process
describes how the hosts join the multicast groups and how the IGMP querier (Router B in
maintains the multicast group memberships:
The hosts send unsolicited IGMP reports to the multicast groups they want to join without having to
1.
wait for the IGMP queries from the IGMP querier.
The IGMP querier periodically multicasts IGMP queries (with the destination address of 224.0.0.1)
2.
to all hosts and routers on the local subnet.
After receiving a query message, Host B or Host C (the host whose delay timer expires first) sends
3.
an IGMP report to the multicast group G1 to announce its membership for G1. This example
assumes that Host B sends the report message. After receiving the report from Host B, Host C
suppresses its own report for G1 because the IGMP routers (Router A and Router B) already know
that G1 has at least one member host on the local subnet. This IGMP report suppression
mechanism helps reduce traffic on the local subnet.
At the same time, Host A sends a report to the multicast group G2 after receiving a query message.
4.
Through the query and response process, the IGMP routers (Router A and Router B) determine that
5.
the local subnet has members of G1 and G2, and the multicast routing protocol (PIM, for example)
on the routers generates (*, G1) and (*, G2) multicast forwarding entries, where asterisk (*)
represents any multicast source. These entries are the basis for subsequent multicast forwarding.
When the multicast data addressed to G1 or G2 reaches an IGMP router, the router looks up the
6.
multicast forwarding table and forwards the multicast data to the local subnet based on the (*, G1)
or (*, G2) entry. Then, the receivers on the subnet can receive the data.
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:
Initially, every IGMPv2 router assumes itself to be the querier and sends IGMP general query
1.
messages (often called "general queries") to all hosts and routers on the local subnet. The
destination address is 224.0.0.1.
After receiving a general query, every IGMPv2 router compares the source IP address of the query
2.
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.
All the non-queriers start a timer, known as an "other querier present timer." If a router receives an
3.
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.
Figure 21, Host B and Host C are interested in the multicast data addressed to the multicast
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Figure 21)