Internet Group Management Protocol (Igmp) - D-Link DGS-3308 Series User Manual

8-port Gigabit Ethernet Switch User's Guide

Internet Group Management Protocol (IGMP)

Computers and network devices that want to receive multicast transmissions need to inform nearby routers that they will
become members of a multicast group. The Internet Group Management Protocol (IGMP) is used to communicate this
information. IGMP is also used to periodically check the multicast group for members that are no longer active.
In the case where there is more than one multicast router on a subnetwork, one router is elected as the 'querier'. This
router then keeps track of the membership of the multicast groups that have active members. The information received
from IGMP is then used to determine if multicast packets should be forwarded to a given subnetwork or not. The router can
check, using IGMP, to see if there is at least one member of a multicast group on a given 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:
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.
Figure 5-15. IGMP Message Format
Type Meaning
Membership Query (if Group Address is 0.0.0.0)
0x11
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 5-11. IGMP Type Codes
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