Enhancements In Igmpv2 - HP 4800G Series Configuration Manual

4) At the same time, because Host A is interested in G2, it sends a report to the multicast group
address of G2.
5) Through the above-mentioned query/report process, the IGMP routers learn that members of G1
and G2 are attached to the local subnet, and the multicast routing protocol (PIM for example)
running on the routers generates (*, G1) and (*, G2) multicast forwarding entries, which will be the
basis for subsequent multicast forwarding, where * represents any multicast source.
6) When the multicast data addressed to G1 or G2 reaches an IGMP router, because the (*, G1) and
(*, G2) multicast forwarding entries exist on the IGMP router, the router forwards the multicast data
to the local subnet, and then the receivers on the subnet receive the data.
As IGMPv1 does not specifically define a Leave Group message, upon leaving a multicast group, an
IGMPv1 host stops sending reports to the address of the multicast group it listened to. If no member of
a multicast group exists on the subnet, the IGMP router will not receive any report addressed to that
multicast group, so the routers will delete the multicast forwarding entries for that multicast group after a
period of time.

Enhancements in IGMPv2

Compared 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 on the same subnet.
In IGMPv2, an independent querier election mechanism is introduced. The querier election process is
as follows:
1) Initially, every IGMPv2 router assumes itself as the querier and sends IGMP general query
messages (often referred to as general queries) to all hosts and routers on the local subnet (the
destination address is 224.0.0.1).
2) Upon hearing 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 other IGMPv2 routers become non-queriers.
3) All the non-queriers start a timer, known as "other querier present timer". If a router receives an
IGMP query from the querier before the timer expires, it resets this timer; otherwise, it assumes 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
router. The multicast router relies on host response timeout to know whether a group no longer has
members. This adds to the leave latency.
In IGMPv2, on the other hand, when a host leaves a multicast group:
1) This host sends a Leave Group message (often referred to as leave message) to all routers (the
destination address is 224.0.0.2) on the local subnet.
2) Upon receiving the leave message, the querier sends a configurable number of group-specific
queries to the group being left. The destination address field and group address field of the
message are both filled with the address of the multicast group being queried.
3) One of the remaining members, if any on the subnet, of the group being queried should send a
membership report within the maximum response time set in the query messages.
1-3