D-Link xStack DGS-3620 Series Reference Manual page 267

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xStack DGS-3620 Series Layer 3 Managed Stackable Gigabit Switch Web UI Reference Guide
subnetwork. If there are no members on a subnetwork, packets will not be forwarded to that subnetwork.
IGMP Versions 1 and 2
Multicast groups allow members to join or leave at any time. IGMP provides the method for members and multicast
routers to communicate when joining or leaving a multicast group.
IGMP version 1 is defined in RFC 1112. It has a fixed packet size and no optional data.
The format of an IGMP packet is shown below:
The IGMP Type codes are shown below:
Type
0x11
0x11
0x16
0x17
0x12
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 6-88 IGMP Message Format
Meaning
Membership Query (if Group Address is 0.0.0.0)
Specific Group Membership Query (if Group Address is Present)
Membership Report (version 2)
Leave a Group (version 2)
Membership Report (version 1)
Figure 6-89 IGMP Type Codes
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