3Com SuperStack 4 Configuration Manual page 168

180 C 7: M
HAPTER ULTICAST
Internet Group
Management Protocol
(IGMP)
P
ROTOCOL
IGMP is a protocol in the TCP/IP suite, responsible for management of IP multicast
members. It is used to establish and maintain multicast membership among IP
hosts and their directly connected neighboring routers. IGMP excludes
transmitting and maintenance of membership information among multicast
routers, which are completed by multicast routing protocols. All hosts participating
in multicast must implement IGMP.
Hosts participating in IP multicast can join and leave a multicast group at any time.
The number of members of a multicast group can be any integer and their
location can be anywhere. A multicast router does not need and cannot keep the
membership of all hosts. It only uses IGMP to learn whether receivers (i.e., group
members) of a multicast group are present on the subnet connected to each
interface. A host only needs to keep the multicast groups it has joined.
IGMP is not symmetric on hosts and routers. Hosts need to respond to IGMP query
messages from the multicast router, i.e., report the group membership to the
router. The router needs to send membership query messages periodically to
discover whether hosts join the specified group on its subnets according to the
received response messages. When the router receives the report that hosts leave
the group, the router will send a group-specific query (IGMP Version 2) to discover
whether there are no members in the group.
Up to now, IGMP has three versions, namely, IGMP Version 1 (defined by
RFC1112), IGMP Version 2 (defined by RFC2236) and IGMP Version 3. IGMP
Version 2 is currently the most widely used version.
IGMP Version 2 benefits from the following improvements over IGMP Version 1:
Election mechanism of multicast routers on the shared network segment
■
Leaving group mechanism
■
Specific group query
■
Max response time
■
Election Mechanism of Multicast Routers on the Shared Network Segment
A shared network segment means that there are multiple multicast routers on a
network segment. In this case, all routers running IGMP on the network segment
can receive the membership report from hosts. Therefore, only one router is
necessary to send membership query messages. In this case, the router election
mechanism is required to specify a router as the querier.
In IGMP Version 1, selection of the querier is determined by the multicast routing
protocol. While IGMP Version 2 specifies that the multicast router with the lowest
IP address is elected as the querier when there are multiple multicast routers on
the same network segment.
Leaving Group Mechanism
In IGMP Version 1, hosts leave the multicast group quietly without informing the
multicast router. The multicast router can only depend on the timeout of the
response time of the multicast group to confirm that hosts leave the group. In
Version 2, when a host leaves a multicast group, it will send a leave group
message.