Igmp Configuration - 3Com 7700 Configuration Manual

IGMP Configuration

IGMP (Internet Group Management Protocol) 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 connected neighboring
routers. IGMP excludes transmitting and maintenance information among
multicast routers, which are completed by multicast routing protocols. All hosts
participating in multicast must implement IGMP.
Hosts participating in multicast can join or leave a multicast group at any time, in
any place, and without limitation of member numbers. 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 which 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 no member exists 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. At present,
IGMP Version 2 is the most widely used version.
IGMP Version 2 boasts the following improvements over IGMP Version 1:
Election mechanism of multicast routers on the shared network segment
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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 required 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
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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 when hosts leave the
group. In Version 2, when a host leaves a multicast group, it will send a leave
group message.
Specific group query
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In IGMP Version 1, a query of multicast routers is targeted at all the multicast
groups on the network segment. This is known as General Query.
In IGMP Version 2, Group-Specific Query is added besides general query. The
destination IP address of the query packet is the IP address of the multicast
group. The group address domain in the packet is also the IP address of the
IGMP Configuration 169