xStack® DGS-3620 Series Managed Switch Web UI Reference Guide
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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 5-1 IGMP Type Codes
IGMP packets enable multicast routers to keep track of the membership of multicast groups, on their respective
subnetworks. The following outlines what is communicated between a multicast router and a multicast group
member using IGMP.
A host sends an IGMP “report” to join a group
A host will never send a report when it wants to leave a group (for version 1).
A host will send a “leave” report when it wants to leave a group (for 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.
The Time-to-Live (TTL) field of query messages is set to 1 so that the queries will not be forwarded to other
subnetworks.
IGMP version 2 introduces some enhancements such as a method to elect a multicast querier for each LAN, an
explicit leave message, and query messages that are specific to a given group.
The states a computer will go through to join or to leave a multicast group are shown below:
Figure 5-66 IGMP State Transitions
The current release of the Switch now implements IGMPv3. Improvements of IGMPv3 over version 2 include:
IGMP Version 3
•
The introduction of the SSM or Source Specific Multicast. In previous versions of IGMP, the host would
receive all packets sent to the multicast group. Now, a host will receive packets only from a specific source
or sources. This is done through the implementation of include and exclude filters used to accept or deny
traffic from these specific sources.
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