User's Manual

Configuration Guide of XGS-5240-Series
42-8
n the received response message, and send out query of specific group (IGMP version
2) 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 IGM
P under this network segment can get membership report message from the host, ther
efore, only one switch is required to transmit membership query message, so an excha
nge 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; I
GMP version2 made an improvement for it, it prescribed that when there are more tha
n one multicast switches on the same network segment, the multicast switch with the l
owest 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 dete
rmine the leave of multicast member by multicast group response time-out. But in versi
on2, when a host decides to leave a multicast group, if it is the host which gives resp
onse to the latest membership query message, then it will send out a message implyin
g 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 net
work segment. This query is called general group query. In IGMP version2, query of sp
ecific group is added besides general group query. The destination IP address of this k
ind of query message is the IP address of the multicast group, the group address field
part of the message is also the IP address of the multicast group. Thus it is prevente
d that hosts which are other multicast group members transmit response message.
4. IGMP version2 added the biggest response time field
IGMP version2 added the biggest response time field to dynamically adjust the res
ponse time of the host to group query message.
The main features of version3 is allowing the host to choose receiving from or reje
cting a certain source, which is the basis of SSM （Source-Specific Multicast）multicast.
For example, when a host is sending a report of INCLUDE{10.1.1.1, 10.1.1.2} to some
group G, that means the host needs the router to forward the flux from 10.1.1.1 and