SNR S2989G-48TX-DC — understanding IGMP Protocol and Multicast Group Management [254/553]

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S2989G-24TX Operation Manual

Chapter 5 Multicast Protocol Related Configuration

4-29

maintain multicast group member relationship between IP host and its neighbor multicast

switches. IGMP does not include the spread and maintenance of relation information of

group members among multicast switches, this work is accomplished by each multicast

routing protocol. All hosts participating in multicast must implement IGMP protocol.

Hosts participating IP multicast can join in and exit multicast group at any location,

any time and without limit of member total. Multicast switch does not need and not likely to

save all relationships of all hosts. It only gets to know if there are receivers of some

multicast group, i.e. group member, on the network segment each interface connects to.

And the host only needs to save which multicast groups it joined.

IGMP is asymmetric between host and router: the host needs to respond the IGMP

query messages of multicast switches, i.e. to report message response in membership;

the switch sends out membership query messages periodically, and then determine if

there are hosts of some specific group joining in the sub-network it belongs to based on

the received response message, and send out query of specific group (IGMP version2)

when receiving the report of a host exiting the group to determine if there exists no

member in some specific group.

Up to now, there are three versions of IGMP: IGMP version1 (defined by RFC1112),

IGMP version2 (defined by RFC2236) and IGMP version3 (defined by RFC3376).

The main improvements of IGMP version2 over version1 are:

1. The election mechanism of multicast switches on the shared network segment

Shared network segment is the situation of there is more than one multicast switch on

a network segment. Under this kind of situation, since all switches which runs IGMP under

this network segment can get membership report message from the host, therefore, only

one switch is required to transmit membership query message, so an exchange election

mechanism is required to determine a switch as query machine. In IGMP version1, the

selection of query machine is determined by Multicast Routing Protocol; IGMP version2

made an improvement for it, it prescribed that when there are more than one multicast

switches on the same network segment, the multicast switch with the lowest IP address

will be elected as the query machine.

2. IGMP version2 added Leave Group Mechanism

In IGMP version 1, the host leaves the multicast group silently without sending any

notification to any multicast switch. This causes that the multicast switch can only

determine the leave of multicast member by multicast group response time-out. But in

version2, when a host decides to leave a multicast group, if it is the host which gives

response to the latest membership query message, then it will send out a message

implying it is leaving.

3. IGMP version 2 added the query to specific group

In IGMP version1, a query of multicast switch is for all multicast groups on the