Chapter 5 Igmp Configuration; Igmp Overview - Huawei Quidway S6500 Series Operation Manual

Operation Manual - Multicast Protocol
Quidway S6500 Series Ethernet Switches

Chapter 5 IGMP Configuration

5.1 IGMP Overview

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 between IP hosts and their directly connected
neighboring routers. The transmission and maintenance of membership information
among multicast routers are completed by multicast routing protocols. All the hosts
participating in multicast must support IGMP.
An IP multicast host can join or exit a multicast group at any time and any where. There
is no restriction on the total number of group members. A multicast router needs not
and cannot save the membership information of all the hosts. It only checks the network
segment connected with each interface by IGMP to see whether there are receivers of
a multicast group, namely group members. A host only needs to save the information
telling to which multicast groups it joins.
IGMP is not symmetric between the host and the router. The host needs to reply, as a
group member, to IGMP query messages from the multicast router. The router needs to
send membership query messages periodically and to check if any host of a specified
group joins to its subnet based on the received response messages. When the router
receives a report on the quit of hosts, the router will send a group-specific query (IGMP
Version 2) to find out if this group still has a group member.
Up till now, IGMP has three versions: IGMP Version 1 (defined by RFC1112), IGMP
Version 2 (defined by RFC2236) and IGMP Version 3. And the popular one now is
Version 2.
IGMP Version 2 excels Version 1 in:
I. Querier selection 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 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.
IGMP Version 1 selects the querier by the multicast routing protocol, while IGMP
Version 2 decides on the multicast router with the lowest IP address as the querier.
II. 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
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Chapter 5 IGMP Configuration