Figure 11-3 IGMP Query-and-Response
As shown in Figure 11-3, Suppose Host B and Host C expect to receive the multicast traffic
sending to multicast group G1, and Host A expects to receive the multicast traffic sending to
multicast group G2. The basic process of the host joining the multicast group and the IGMP
querier (Router B) maintaining the multicast group membership is as below:
(1)
Instead of waiting for the IGMP query message from the IGMP querier, the host will actively
send IGMP membership report message to the multicast group it wants to join in.
(2)
The IGMP querier will periodically send the IGMP query message to all the hosts and
routers in the local network with the multicast address 224.0.0.1.
(3)
After receiving the IGMP query message, the host that is interested in multicast group G1,
either Host B or Host C (depending on whose latency timer runs out first) — for example
Host B, will firstly multicast IGMP membership report message to G1 to declare it belongs
to G1. As all the hosts and routers can receive this membership report message and the
IGMP routers (Router A and Router B) already know there is a host interested in G1, Host C
will not send its report message for G1 after it receives the report message of Host B. This
is called the membership report preventing mechanism and it helps to reduce the traffic in
the local network.
(4)
At the same time, as Host A is interested in G2, it will multicast report message to G2 to
declare it belongs to G2.
(5)
Through the above query-and-response process, the IGMP router learns that there are
group members of G1 and G2 in the local network. It will generate the multicast forwarding
entries (*, G1) and (*, G2) via the multicast routing protocol, such as PIM, as the basis of the
multicast traffic forwarding. The symbol * represents any multicast source.
(6)
When multicast packets sending to G1 or G2 from the multicast source arrive at the IGMP
router via multicast routing, the multicast forwarding entries (*, G1) and (*, G2) in the IGMP
router will guide the multicast packets to the local network and the receiver hosts can
receive them.
258
