D-Link DES-3226S User Manual page 43

The IGMP Type codes are shown below:
Type Meaning
0x11 Membership Query (if Group Address is 0.0.0.0)
0x11 Specific Group Membership Query (if Group Address
is Present)
0x16 Membership Report (version 2)
0x17 Leave a Group (version 2)
0x12 Membership Report (version 1)
Table 3. IGMP Type Codes
Multicast routers use IGMP to manage multicast group memberships:
•
An IGMP "report" is sent by a user's computer to join a group
•
IGMP version 1 does not have an explicit 'leave' message. Group members have an expiration timer, and if this timer
expires before a query response is returned, the member is dropped from the group.
•
IGMP version 2 introduces an explicit "leave" report. When a user wants to leave a group, this report is sent to the
multicast router (for IGMP version 2).
•
Multicast routers send IGMP queries (to the all-hosts group address: 224.0.0.1) periodically to see whether any group
members exist on their subnetworks. If there is no response from a particular group, the router assumes that there are
no group members on the network, and multicast packets are not forwarded.
The TTL field of query messages is set to 1 so that the queries do not get forwarded to other subnetworks.
IGMP version 2 introduces a few extensions to IGMP version 1 such as, the election of a single multicast querier for each
network, explicit 'leave' reports, and queries that are specific to a particular multicast group.
The router with the lowest IP address is elected as the querier. The explicit group leave message is added to decrease latency,
and routers can ask for membership reports from a particular multicast group ID.
The transition states a host will go through to join or leave a multicast group are shown in the diagram below.
DES-3226S Layer 2 Fast Ethernet Switch User's Guide
Figure 5 - 9. IGMP State Transitions
36