User`s guide

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Figure 2-63
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Figure 2-64
2.6 Multicast
2.6.1 IGMP Snooping
 Theory
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IGMP is also used to periodically check the multicast group for members that are no
longer active.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 sub
network, one router is elected as the ‘queried’. 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 sub network or not. The router can check, using IGMP, to see if
there is at least one member of a multicast group on a given subnet work. If there are
no members on a sub network, packets will not be forwarded to that sub network.
 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.