3Com Switch 7754, Configuration Manual

Page: 450 / 940

450 C
HAPTER
46: IGMP C
ONFIGURATION
In IGMP Version 1, the multicast routing protocol selects the querier. In IGMP
Version 2, 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.
Leave group mechanism
In IGMP Version 1, 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 IGMP Version 2, 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.
Group-specific query
In IGMP Version 1, a multicast query message of the multicast router aims at all
the multicast groups in the network segment. This query is called general query.
IGMP version 2 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.
Maximum response time
The Maximum Response Time field is added in IGMP Version 2. It is used to
dynamically adjust the maximum time for a host to respond to the membership
query message.
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 group member 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.
All the receiver hosts participating in multicast transmission must support the
IGMP protocol. The multicast router need not and cannot save the membership
information of all the hosts. It 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 that which multicast
groups it joins.
Working mechanism of IGMPv1
IGMPv1 protocol (RFC1112) manages the multicast groups based on the
query/response mechanism. With the help of the Layer 3 routing protocol, IGMP
selects the designated router (DR) as the querier, which is responsible for sending
query messages. Figure 110 describes the IGMPv1 message interaction in the
network: