Working Procedure Of Igmp - H3C S7500 Series Operation Manual

Operation Manual – Multicast
H3C S7500 Series Ethernet Switches
membership query messages. In this case, the querier selection mechanism is
required to specify a router as the querier.
In IGMPv1, the multicast routing protocol selects the querier. In IGMPv2, it is defined
that the multicast router with the lowest IP address is selected as the querier when
there are multiple multicast routers in a network segment.
II. Leave group mechanism
In IGMPv1, hosts leave the multicast group quietly without informing any multicast
router. Only when a query message times out can the multicast router know that a host
has left the group. In IGMPv2, when a host replying to the last membership query
message decides to leave a multicast group, it will send a leave group message to the
multicast router.
III. Group-specific query
In IGMPv1, a multicast query message of the multicast router aims at all the multicast
groups in the network segment. This query is called general query.
IGMPv2 adds group-specific query, where the IP address of a multicast group is taken
as the destination IP address and the group address field of the query message, to
prevent the member hosts of other groups from responding to this message.
IV. Maximum response time
The Maximum Response Time field is added in IGMPv2. It is used to dynamically
adjust the maximum time for a host to respond to the membership query message.

6.1.3 Working Procedure of IGMP

The working procedure of IGMP is as follows:
The receiver host reports the membership to its shared network.
A querier (IGMPv2) is selected from all the IGMP-enabled routers in the same
network segment.
The querier periodically sends query messages to the shared network segment.
The receiver host responds to the received query message to report the group
membership.
The querier refreshes the presence information of the group members according
to the received responses.
IGMP must be enabled on all the multicast receiver hosts. An IP multicast host can join
in or exit a multicast group anywhere and anytime, without being restricted by the total
number of group members.
The multicast router need not and cannot save the membership information of all the
hosts. It just checks the network segment connected with each interface by IGMP to
see whether there are receivers of a multicast group, namely, group members. While
each host saves only the information about the multicast groups it joins.
6-2
Chapter 6 IGMP Configuration