Planet XGS3-42000R User Manual page 395

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 s witch as query machine.
In IGMP version1, the selection of query machine is determined by Multicast Routing Prot ocol; 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 respons e 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 network segment. This query
is called general group query. In IGMP version2, query of specific group is added besides general group
query. The destination IP address of this kind 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
prevented 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 response time of the host to
group query message.
The main features of version3 is allowing the host to choose receiving from or rejecting 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 10.1. 1.2; when a host is sending a report of E XCLUDE{192.168. 1.1} to some group G, that
means the host needs the flux from all sources of group G except 192.168.1.1. This makes a great difference
from the previous IGMP.
The main improvements of IGMP Version3 over IGMP Version1 and Version2 are:
1. The status to be maintained is group and source list, not only the groups in IGMP v2.
2. The interoperations with IGMP v1 and IGMP v2 are defined in IGMP v3 status.
3. IP service int erface is modified to allow specific sourc e list thereby.
4. The queried includes his/her Robustness Variable and Query Interval in query group to allow the
synchronization with these variables of non-queries.
5. Max Response Time in Query Message has an exponential range, with maximum value from 25.5
secs of v2 to 53 mins, which can be used in links of great capacity.
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