3Com Switch 7754, Configuration Manual

Page: 449 / 940

46
IGMP C
ONFIGURATION
Overview
Introduction to IGMP Internet group management protocol (IGMP) is responsible for the management
of IP multicast members. It is used to establish and maintain membership between
IP hosts and their directly connected neighboring routers.
The IGMP feature does not transmit and maintain the membership information
among multicast routers. This task is completed by multicast routing protocols. All
the hosts participating in multicast must support the IGMP feature.
IGMP is divided into two function parts:
■
Host side: the hosts participating IP multicast can join or exit a multicast group
anywhere and anytime.
■
Router side: through the IGMP protocol, a multicast router checks the network
segment connected with each interface to see whether there are receivers of a
multicast group, namely, group members.
A multicast router need not and cannot save the membership information of all
the hosts. While, a host has to save the information that which multicast groups
that it joins in.
IGMP is asymmetric between the host and the router. The host needs to respond
to the IGMP query messages of the multicast routers, that is, report message
responses as an IGMP host. The multicast router sends IGMP general query
messages periodically and determines whether any host of a specified group joins
in its subnet based on the received response packets. When the router receives
IGMP leave messages, it will send IGMPv2 group-specific query messages to find
out whether the specified group still has any member.
IGMP Version IGMP has three versions until now, including: IGMP Version 1 defined by RFC1112,
IGMP Version 2 defined by RFC2236 and RFC Version 3. IGMP Version 2 is the
most widely used currently.
Compared with IGMP Version 2, the advantages of IGMP Version 2 are:
Multicast router election mechanism on a shared network segment
A shared network segment is a network segment with multiple multicast routers.
In this case, all routers running IGMP on this network segment can receive the
membership report messages from hosts. Therefore, only one router is necessary
to send membership query messages. In this case, the querier selection
mechanism is required to specify a router as the querier.