H3C S7500E Series Ethernet Switches Operation Manual

Operation Manual – Multicast
H3C S7500E Series Ethernet Switches Chapter 4 IGMP Configuration
4-3
4) Host C, which is on the same subnet, hears the report from Host B for joining G1.
Upon hearing the report, Host C will suppress itself from sending a report
message for the same multicast group, because the IGMP routers (Router A and
Router B) already know that at least one host on the local subnet is interested in
G1. This mechanism, known as IGMP report suppression, helps reduce traffic
over the local subnet.
5) At the same time, because Host A is interested in G2, it sends a report to the
multicast group address of G2.
6) Through the above-mentioned query/report process, the IGMP routers learn that
members of G1 and G2 are attached to the local subnet, and generate (*, G1) and
(*, G2) multicast forwarding entries, which will be the basis for subsequent
multicast forwarding, where * represents any multicast source.
7) 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 that the host will leave. 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.
4.1.3 Enhancements Provided by IGMPv2
Compared with IGMPv1, IGMPv2 provides the querier election mechanism and Leave
Group mechanism.
I. 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